I posted my math on this on my blog. The post is a little long but it shows my method for figuring it out.  Of course (since I ordered a BFL 50ghs) the whole thing assumes that the butterfly lab units will eventually ship. I think they will. If they don't the hash rate won't change much. But if they do ship it changes a LOT.

http://mineshaft.me/2013/04/initial-roi-estimates/