Pretty much the same "rules" apply for buying a classical guitar as any other. The quality of cheap production-line guitars is blurring the lines between top and bottom end products. As you're wanting something to use at festivals, would the guitar you choose need to have built in pickups? Nylon strings don't project anywhere near as well as steel and you would need a top quality microphone to get the sound out. Easiest choice is a guitar pickup but that's where buying cheap WILL let you down. There are some good electro-accoustics that include a nylon string in their range, Yamaha APX for example, which have all the electronics included and are designed (neck-wise) for the non-classical player. Not exactly "period-correct" I know, but it's wired up discreetly. If all you're needing is a box and amplification isn't an issue, then there's tons of good, cheap guitars around. Just one tip... use good strings!