Don't forget there is a minimum tread depth by law that makes it illegal to run 'bald' tires (varies by local me thinks). Get in an accident with legally 'bald' tires, and you could be liable even if tires weren't really a factor.

Just something to keep in mind. Penny wise pound foolish would seem to apply here. Trying to eek out every cent of tread life can easily cost you big. Better to get new tires before the old ones are completely used up.