One thing that has kept me from using more green stuff in the past has been the price. Games Workshop charges about $8 for 6" of the stuff. If you are going to be doing a lot of sculpting that can add up. Especially if you aren't any good at it and go through a bit of green stuff.
But don't worry - I have found a solution to this problem. Green stuff is actually Kneadatite which is manufactured by Polymerics Systems, Inc. There are several online resellers where you can get 36" of green stuff for under $20 shipped.
Last time I bought some green stuff I picked it up at greenputty.com. It was on sale and I managed to get a 36" roll for about $15 shipped.
So next time you need some green stuff do a bit of searching on the internet and I bet you can beat GW's prices.