In the end the hat was sold for 9 keys and I've found people interested in buying keys for 850 ce each. However, I'm probably going to keep a few keys.

I probably should have contacted you first Hitten as to give you first dibs on a price of 9 keys, but everything moved quite quickly when I began poking around the TF2 forums with an asking price of 9 keys. I pretty much posted, got responses, traded, and it was all done within 15 minutes.

Some other miscellaneous things I'd like to note was that the site I had linked above tends to overprice thing so only use it as an advisory price gauge. I've been seeing painted versions go for 10 keys so it didn't seem like an unpainted one would or should go for as much. Going into the TF2 trading servers was probably the fastest way to get an idea of how much it's worth and then posting on the TF2 trading sub-forum to see if there is any interest. Trading TF2 items within TF2 for keys and refined and then selling those two items for ce is probably the best route to go for these kind of items.

So in short I suppose a Bill's Hat is worth 7650 ce in Spiral Knights.