Most expensive single-item purchase is definitely the double Ferret Nation cage I just got my girl. But it was worth it, we both love it! She has more space with more shelves and platforms to bounce around on, and it's SO much easier for me to clean than her old cage. Those dual-opening doors are so awesome!
That said, I've probably spent at least as much as I did on the cage (actually, probably more) on all her various accessories. Shelves, liners, hammocks, chew toys, a playpen....it all starts to add up real quick.