an automotive machine shop will usually charge a preset price according to the work required. A list for head work might include:
clean heads
mill heads
angle mill heads
cc heads
standard valve job
competition 5-angle valve job
R/R valve guides
hone valve guides
install seats
port matching
full port work
I'm sure there are other things I'm not thinking of, but you get the idea. Each task will have a preset amount. Parts are always extra.
For most garage work (ie. replace throwout bearing), they look up the project in a manual that tells them approximately how much time the job should take if all goes well. That'll be your rough estimate for their work. When it's all done, they'll take the actual amount of time spent on the job and add it up $$$/hr. Once again, parts are always extra.
Sorry I can't give you a better idea of how much $$$ it'll be, but the only people that touch my cars other than me, are those who spray paint and those who do wheel alignments.
Good luck.