"Flowing" a system, generally involves the complete "blueprinting" of the system, including the plate, solenoids and any other parts, to make sure they deliver the best performance, are assembled correctly and operate correctly. Then the components are actually sprayed on the flow bench and jet mapped, to get the actual lb/hr numbers and set the tune up ratios. As far as prepping a plate, to actually try and "fix" the corners on something, that would be virtually impossible. How the N/F mixture flows through a static intake and how it does on a running engine, could be two entirely different situations. What is achieved, is to make sure the plate operates correctly, has a good pattern and get the numbers. A "correct" tune, will go a long way toward correcting the corner issue, that many seem to think is a problem.
As far as cost, that is something that will vary from shop to shop, as well as be dependant on the parts you have, the intended useage of the system and other things. You will just have to choose who you want to do it and give them a call.