What is the cost of owning a Flemish Giant rabbit?

Hi
We are considering adopting a baby Flemish Giant, however we are wary of the cost as I've read they eat so much more than other breeds. I'm not worried about "start up" costs, such as housing and whatnot, just food really. He/She will be an indoor rabbit. I've found many listings for Timothy hay with prices on Amazon however, I have no idea how much I'll need to buy and how often. Also I'd rather buy in bulk but I don't want it going bad.....how long does Timothy hay last?