Multi-bay enclosures that take SATA drives are expensive these days. If you can settle for IDE, you'll be able to find them at more reasonable prices. I know - I recently got bit by the same issue. I'm not particularly pleased that I had to go with the more expensive enclosure, but I didn't have much other choice.
The problem is that the controllers for these cases are the lion's share of the cost, and you need more controllers as you increase the number of hard drives. That's why it doesn't get any cheaper as you get more drives. Factor in the higher cost of the SATA-capable controllers that can handle two drives at a time, and you know why things are so expensive.
If you're the modding sort, you can buy an appropriately-sized case from Directron with no controllers and then wire it up with eSATA using the appropriate cables, a dremel tool, and some superglue. Make sure the cables are VERY secure, though - SATA doesn't take well to jostling.