Make sure you know how much your overall budget is, and then accumulate statistics on what happens in your library. List all transactions, and count them. Hopefully, you already have these statistics for the last year or so. Then divide the number of service points into the budget for a cost per service point. The average researcher cost for each organization must be developed.
Most regular users indicate savings of hundreds of hours per year.
The library is only adding .74 percent to the cost of the researcher's salary. That is a very small figure. It should be easy for you to get the support for these figures from a user survey. Find out how important the library is to researchers' jobs. Find out how many hours they save per year by using the library.