yah opto's are doable, the slow ones are like $0.20 a piece, if you can live with a slower baud rate they are fine.
http://www.mouser.com/ds/2/239/775704_1-268944.pdf
Faster ones cost a bit more and have larger power requirements.
FYI, in the "one cpu per cell" model I am working on only using them on the ends of the battery pack, and node to node communication uses a level shifter. But per cell cost is important, and it should be fast and still operate within the expected voltage range.