A 2003 DOE/National Center for Educatation Statistics Study focused on instructional spending at 4y colleges and universities. Many factors contribute to direct instructional costs. A key finding of this study was that “most of the variance in instructional cost across institutions, as measured by direct expense per student credit hour taught, is associated with the disciplinary mix within an institution.” A secondary factor is “institutional mission, as related to Carnegie institutional classification”. The study found for example that it cost $112 per credit hour to teach English at a “comprehensive university” but $379 per credit hour to teach mechanical engineering at a “research university”.
https://nces.ed.gov/pubsearch/pubsinfo.asp?pubid=2003161
As far as I know, the public data available from the DOE/IPEDS does not break out salaries (or other components of instructional spending) by discipline.