Depends on the problems you have - new air springs are about £90 a piece I think, but if its the compressor thats gone belly-up then it's a good few quid.
I've not had any probs with my SLS, and find it really useful. If (when ?) the airbags start to leak I'll replace them, but if the compressor packs up, then I'll have to think about coverting to traditional coils.