cheaper 580 EX II for wireless use with ST-E2
I have a Sigma DG530 Super (canon version) that works with my 580 EX II as a master and they're fairly inexpensive. I don't know if it will work with the ST-E2, but I'll bet a google search will find posts from folks who have tried. I can say it works poorly as a master with 580 EX II as slave, very intermittent. I suspect it would work well with other DG530's as slave.