I am looking for good alternative to a concrete slab in my basement.  I have seen a system with rigid foam and steel studs that you have put over a compacted level gravel base and then plywood over that, but the claims of being cheaper than a concrete slab were off, so am wondering if anyone has any other ideas or if one could do the same thing themselves with rigid foam and some type of stud.  
regards

bbicf;

it is possible to use AAC floor planks, but if cost is the driving factor, slab on grade is  likely the most cost effective

Try using a EPS Foam RAFT Foundation

Which aspect are you trying to improve?

All of them;-)

The slab you are suing right now i think is the best slab according to time period but if you want to change this i will recommend you to go for some wooden slab.