I've done it on a couple of pistols--mostly because I was bored. I can't say that it helped or hurt.
Cost was just some time and patience because I did it myself.
It's not a worthwhile investment if the gun shoots ok as it is, and if it's inaccurate, there are a number of other places I'd start looking in a pistol before I started worrying about the crown.
If there's obvious damage to the crown I'd probably go ahead and recrown it, but just because it would bother me if I didn't.
In a rifle it's more of an issue.