Another thing to consider aside from the USB miners being new: from what I've read, most consider the USB miners to be great for early adopters, hobbyists, enthusiasts, and those who want an ASIC now. Those people are willing to pay a premium for a device that most agree will not return on its investment. So a pair of used FPGA's need to be priced much lower than the equivalent Mh/s/$ of the USB miners.

Another thing to keep in mind, a GPU rig can be built for roughly $1,500 USD for 2.4 Gh/s. At that rate, the 450 Mh/s devices would be priced at around $280. Now factor in the fact that they are used, but also have a much lower power draw than a GPU miner.