Buy 2 devices, mine solo with the first, mine against a pool with the second.
Not everything is about ROI ...

Excellent. I'd like to buy some SP35s below cost.

Out of curiosity, what's our cost ?

A few people on alibaba.com are selling sp20's below $460 in bulk with warranty.
http://www.alibaba.com/product-detail/Spondoolies-Tech-SP-20-Jackson-1_50006306935.html

so somewhere below that.

there are many dubious sellers on alibaba. if you were to send them that $460 you'll find out.

EDIT; min order is 20 units on that particular link