AnandTech has published a roundup of solid state disks that cost around $100, you can check it out over here. The site concludes it's still better to spend $300 to $400 on a SSD, stating that these value SSDs are only worth considering for an older machine or a netbook.
While OCZ and Kingston trade blows below $100 I'd give the overall nod to Kingston's SSDNow V Series Boot Drive. The 30GB drive is limited in capacity but provides very consistent performance, even under completely unrealistic workloads. You don't get great random read/write performance but you do get good overall performance. If you're building a machine for light workloads like web browsing then the Kingston drive should be sufficient. If you're going to be doing a lot of multitasking and have a heavier workload, the OCZ Onyx is a better bet below $100.