Bureau of labor statistics keeps and publishes salary info. The hard part is making the job titles line up.

https://www.bls.gov/bls/blswage.htm

Somewhere on that site there's a listing for H1B levels as well. I saw it on Blind last night but I can't find the thread this morning.

EDIT: Check out this site: https://www.flcdatacenter.com/caseh1b.aspx