Years ago I ran into a guy at the game store with an awesome 100% custom sculpted Nurgle army. I was like WoW thats a lot of greenstuff. Turns out he didn't use any instead he turned me onto a great alternative...
http://www.allfixepoxy.com/servlet/StoreFront
All-Fix this stuff has a working time pretty much the same as greenstuff. However it cures much much harder, and sculpts much easier. I've used it extensively now for a number of years, and just wanted to share the info. I've used it for everything from sculpting little bolts, and ornamentation, to building entire structural items. It carves, sands, perfectly.
The biggest upside is if you use a lot of epoxy putty it's cheap. It's kind of gray, and dark gray. So you don't get the whole yellow, and blue make green effect. However the formula is simple. Take out a little of part A, and roll it into a ball. Take out a little of part B and roll it into a ball. Add/remove, putty from part B until the ball is the same size. Mix thoroughly.
It's very very forgiving on the mix ratio, and I've never had a problem. Off either way it will only slightly increase/decrease the cure time.
It dries to a bone white color, and takes paint perfectly. I got the 12oz kit a couple of years ago, and I've used about half of it so far.
Just thought I would share for anybody looking for a great cheap greenstuff alternative.