U.S. State Probation Rates
While there is no preexisting information to fully answer your question, I've used the readily available data to estimate an approximate figure for the average cost per person in relation to state prison probation in the US. Using a limited number of states as a representative sample, I estimate the daily cost to be around $5.50 consisting mostly of staffing costs. Below you'll find an outline of the research methodology used to better understand why the information you've requested is publicly unavailable, and how this estimate was triangulated.
An extensive search of the public domain revealed no up-to-date data in relation to the average cost of probation in the US for state prisons. Although I found a report on Prison Policy website dated 2017 that referenced “probation costs per year per person”, this was based on information from the Pew Research Center, from a survey conducted in 2008. I searched the website of the Pew Research Center to determine if any update to this survey had been made in the last 10 years, but determined this was the first and latest occasion when this survey was conducted. Pew had collated information for 33 states which provided breakdown of average costs of probation, parole and incarceration. I continued my search for more current information and found that when compiled on the national level, this information tended to focus on the cost of incarceration rather than probation or parole, as was the case for a report from Vera, the Institute of Justice, which looked into state spending on prisons. Total annual prison costs and cost per inmate information from 2015 was provided for 44 states, but did not provide any information on probation costs. A search of the Bureau of Justice Statistics determined that it too does not contain the requested information.
I did, however, find that the National Institute of Corrections has a database of 2015 State Statistics, which also provides cost per inmate by state. Unfortunately again this refers only to the expenses incurred for those incarcerated, and cost per offender on probation is not detailed. The National Institute of Corrections does provide links to the reports of each state from which the cost per inmate figures have been calculated, but I found that in relation to probation, these figures were generally not directly available from each source and would require a number of calculations using figures extracted from each report, which varied for each state. It was therefore not feasible to attempt to calculate each state’s probation cost per offender using those reports to determine the exact national average.
A decade ago, Pew Research determined "[t]he average cost of managing an offender in the community...[was] $3.42 per day for probationers...or about $1,250...a year." This was the average when considering probation agencies only. Some states combine their parole and probation functions into a single agency, and the average cost per person for these states was $3.90. The report that Pew Research published also detailed the low and high end for each of these: for probation agencies, the lowest was $1.38 and the highest was $7.89. For probation and parole agencies, the lowest cost per person per day was $1.22 and the highest was $9.76. Unfortunately, Pew Research have neglected to update their research since this first survey was conducted in 2008. We would assume that costs would rise, but there is no evidence to confirm whether this would be in line with inflation or at some other rate.
Although my search of media reports, government databases and prison-related studies did not return a recent national average for probation costs, I did come across some at the state level which were directly reported. These were North Carolina ($5.31 per person per day), New Mexico ($7.58 per person per day) and Georgia ($4 per person per day).
As discussed in the methodology above, the National Institute of Corrections did provide links to the sites and reports for each state’s department of corrections or equivalent. I selected a sample of states to calculate the state probation cost per person, to then average and use as an estimate for the national probation cost. In order to provide a representative sample, I endeavored to select a low-end figure, a high-end figure and a mid-range figure. As information for cost per inmate was readily available, and assuming some correlation between the cost per incarcerated person and the cost per person on probation, I selected Kentucky which had the lowest cost per inmate, Colorado with a mid-range cost per inmate and New York with the highest cost per inmate for 2015. However, when I searched the site for New York Department of Corrections and Community Supervision, I was unable to locate information about the cost of probation so attempted the next highest state, New Jersey. Unfortunately I couldn’t locate this information for New Jersey, nor for Connecticut, the state with the subsequent highest cost per inmate. I have therefore included Alaska, which was the highest cost state for which the probation cost per person could be determined.
For the financial year 2016, "Judicial Appropriation by Major Function, Probation" was $144,138,492
Given the probation population of 78,883
Annual cost per person = $1,827.24
/ 364.75 = $5.01 per day
For the most recent year available, 2014 the "Appropriation: Population Management, Allocation: Statewide Probation and Parole" was $15,490,800
Given the probation population of 5,418
Annual cost per person = $2,859.14
/ 364.75 = $7.84 per day
The average of these three states = ($3.46 + $5.01 + $7.84)/3= $5.44 per person, per day
Including the three states for which I was able to locate directly reported daily costs of probation (North Carolina, Georgia and New Mexico), which can be considered a haphazard selection, in addition to the three states chosen to represent low to high range of costs (Kentucky, Colorado and Alaska), the average remains broadly the same at $5.53 per person, per day. Based on this limited sample, the average national probation cost seems to be in the region of $5.50.
($5.31 + $4.00 + $7.58 +$3.46 + $5.01 + $7.84)/6= $5.53 per person, per day
There was limited explanation provided in any of the reports from which these figures were extracted regarding what these costs consisted of. An article from the US Courts regarding cost of probation states that it includes "salary costs of probation and pretrial services officers, obligations for expenditures associated with providing court-directed services for defendants and offenders, and miscellaneous operating expenses". Although this is in relation to the federal system and dates back to 2013, it is likely applicable to the state system in the current day. An extract from the book “Community Corrections” notes that typical costs include staffing with roles of "Chief Probation Officer, Senior Probation Officer, Probation Officers, Accounting Technician and Admin Secretary" plus “rent, training, vehicle replacement and other mundane costs”, where “mundane costs” is likely to be equivalent to general and administration costs that any normal office would be expected to incur. A more granular breakdown of expenses that make up the total cost of probation was not available.
Despite the limited data available, I was able to estimate an average US state probation cost per person to be around $5.50 per day, consisting mainly of salaries and the typical expenses that are incurred when running an office, such as training and rent.