Mad you have more than enough resolution to print up to 20x30. I can suggest going higher as i never did it myself. So that's up to you to try.
As far as getting it printed, you can go to a good photolab, or try one of the online places. There's a ton of places like snapfish, shutterfly, etc that print up to that size. The difference is the type of paper they use that will yield better results. So it's a good idea to check out which is one of the better online printlabs.
I suggest that if you do get it printed online that you get it framed asap. The paper sometimes wrinkle fast if it's too thin and can end up getting ruined. So getting it framed professionally is something i recommend.
In terms of prices, i wouldn't even worry about that. If it's a picture you truly love, then it should be priceless!
