I used to get the .. c3.xlarge (l8 cores, 10GB RAM?... the "budget" CPU server).. I guess maybe I shouldn't say this if it's ever profitable again, but you could always get those @ spot price for .07c in US East (I had 100 running on multiple occasions).
At .07 how long did you typically retain the server for? Just 1 hour or multiple hours? How do you setup the storage so you don't lose the data when the spot ends?