Its a hard call. They'll need to spend money in replacing him and they don't need a striker if they keep Tevez/Ballo/Aguero. Sneijder could be a good buy and shouldn't cost more than 20m. but everyone else of the same or higher quality will cost more than 30m.
I think Sneijder would probably cost more than that and he's a bit older on top of that so we'd end up having to replace him sooner than Silva in the end. On top of that, I don't think he's anywhere near as good as Silva.
It'd be fun in Football Manager kinda way to see what we'd do but that's about it.