There are a number of 12x24 frames on amazon that are around $20 apiece. I'm not sure you're going to get much cheaper than that. Also, keep in mind that shipping costs will go down when you're buying in bulk. Otherwise, Michael's can have some good deals on frames.

Example: ArtToFrames FRBW26079-12x24 Satin Black Picture Frame, 1-1/4-Inch Wide