With recent, major price drops on the cost of solid-state storage, even high-capacity USB 3.0 external SSD drives are within reach of even a budget conscious consumer. With aggressive pricing, LaCie's 120GB solid-state portable drive is now prices under $100 USD. Larger capacities offer even lower cost-per-gigabyte for USB enabled Apple computers old or new.
For the most affordability, the best external SSD drive for your Mac laptop or desktop is a USB 3.X solid-state drive. Backward compatible with slower USB 2.0 (and even USB 1.1 ports when needed) they offer low-cost opportunities to step into the fast lane of SSD storage. And, at least with single-drive solutions, they offer comparable data transfer rates to ThunderBolt without the additional expense that buying a ThunderBolt drive entails.