ported polished and milled

Hey, I'm looking into engine work that I can afford but want a better understand of my options before I spend my cash. Now I have a very general understand of these processes and I need a good explaination of what is done, what kind of gains to expect, and the price range. I was looking into porting and polishing, milling, and valve jobs. Any information on similar processess that I haven't mentioned would be appreciated as well.