looks like a Synodontis of some sort. 6"+ go for about $20-$30 at my lfs. very mean cat. if you look on the inside of it's lips they have rows of sharp little teeth. they also like to scrap. good luck with your new little terror!
i have one which i bought for 30bucks .....by mistake though, but overall it likes to hide during the day n pretty active during night. mine get along well with poly, texas, royal pleco n two parrots.