Hmm...

Presently at Slush I get approx. 0.002BTC@~600MH/s at each block (averaged). We find around 16-18 blocks per day.
Assuming I were to pay 2.2BTC for this usb 'block eruptor' I would be looking at:

2.2 / 0.001 / 17 = 130 days to break-even, assuming network difficulty remains as-is, which it WILL NOT.

Because estimates of rate of difficulty increase vary wildly I hesitate to _speculate_ further, but it seems to me that there is a possibility this buy will not pay for itself in a reasonable time frame.