For $100, you're not going to get a new card. New hardware RAID cards typically run several hundred dollars. What you can get is an old server-class RAID card pulled from a retired system.
I recommend the Areca ARC-1120 8-port card. It's a universal PCI/PCI-X card, so it can fit in almost any PCI or PCI-X slot (the exception being those few PCI mainboards with components blocking the area occupied by the PCI-X part of the connector). It supports RAID 5 (as well as a variety of other RAID levels), supports up to 8 SATA hard drives, doesn't require any drivers under Linux for basic operations but has them (as well as management software) for advanced functionality, and you can usually find one on Ebay for under $100. Two things to watch out for when buying off Ebay: people sometimes mislabel the ARC-1120ML (two SAS multilane connectors rather than eight SATA connectors) or the ARC-1110 (four SATA connectors) as the ARC-1120.
I can also strongly dis-recommend the 3Ware 9500S-8. On paper, it looks similar to the ARC-1120, but appears to be optimized for benchmarking, and performs quite poorly with small-random-write workloads.