You can spend as much as you want for an acoustic...but you can do well with a used one in the $100-200 range...My daughter has a sweet little Alvarez Dreadnought that I picked used up on craigslist for $75.. Think its the RD8..not a fancy guitar but it plays well, sounds good, has decent tuners, and it stays in tune.

Whatever you get...take it to a guitar repair shop and have them look at it.....Youll want them to check the neck relief, and the string height at the nut and bridge and adjust it if neccesary. A good set up is money well spent. Cheers.