It depends how good of a port job it is. Porting an iron head isn't a quick and easy job, Find out what's this person is going to do for $200. I've heard of people just grinding down the EGR bumps on the exaust side for some pretty decent gains, you can do that yourself. I have Thumper Ported E7's and I paid a total of $700 and couldn't be happier. Well, I could but of course but I got my money's worth.
probably about 20 hp give or take. i've seen some larger vlaved heavily ported ones advertised at 35 to 40 horse but thats with bolt ons and such. usually on a good port job their will be alot of bowl work done and contouring of the valve guide boss, not just the ports being enlarged.