Finally found (on ebay, of all places) a source for cheaper rechargeable lithium ion 2450 size batteries.  At 95 cents each, that is less than 1/2 the cost of prior source: http://goo.gl/HYa7EU, and brings the project cost below the sacred threshold of $10 per tinybot (not including the cost of component shipping from Mouser, although even then the cost would only be a hair more than $10 if you build 10 at a time).