If you're looking for <$25, there is the SuperCard miniSD. I got mine on eBay from China for $18 USD shipped, and I've had NO problems with it. Bare in mind though, it takes miniSD. That's the standard that is smaller than Normal SD, but bigger than microSD. You will likely be able to find a 2 GB miniSD, which is plenty for GBA, GBC, and GB games, for $5-8 from China shipped. There aren't that many places in the US that sell them, because they quickly were replaced with microSD. The good thing is, the miniSD I bought, AND the SuperCard were BOTH tracked from acceptance to my mailbox.
Truthfully though, I wish I shelled out the extra cash for an everdrive or EZ-omega. While I have had no problems with the supercard, it is lackluster, such as having to manually save to the SD card, plus I don't think it has an RTC, which isn't good for games like Pokemon. Also, the SuperCard can only support a 2 GB SD card at max. The everdrive I know, can support up to 64 GB, which IS overkill for the GBA, and all you need at the most is 8 GB, if that.