For me in '13, the start, finish and checkpoint hotels were more expensive than the hotels I used during the event. You don't have to stay at the event hotels, but it's a lot easier if you do. Some riders will get to the start a couple of days early, stay in a M6 or other budget hotel, then move to the start hotel the night before the start so they can do the banquet, go strait to a room to plan and not have to worry about moving until they leave. bikes need to be in impound that night too, complicating things if you're not staying at the start hotel.
I rode to PA from UT, so 3 hotels on the way, 3 nights at the start hotel, one night at the checkpoint hotel, then another night at the finish hotel, plus 9 hotels while riding the IBR. So not counting the trip home, that's 16 nights probably running ~$1500 just for that. Gas was easily $1100 for the total trip including riding to the start and home after. Food was mostly gas station food during the rally, but sit down food before the start at the captured hotel prices wasn't cheap and I had one order in night during the rally when I screwed up and had to park and wait until the Henry Ford museum opened the next day, which cost more than if I'd have been moving. I'll just round up and say $500 for food. That's $3100.
Then there are the other expenses. A rear tire, plus shipping it to a shop at checkpoint two, the cost to have it M&B there. The pair of new tires I had drop shipped to a shop in PA for the start and the cost to M&B those. (I R&R'd my wheels both times. Oil change before the start, (Walmart parking lot), oil change after the finish, (Walmart parking lot), extra set of brake pads, (just in case).
Spot costs if you don't already have it. AAA and/or other recovery service dues, if you don't already have it/them. MedJet dues, if you don't already have it.
The IBR can be done cheaper or more expensive. All in, not counting the entry fee or bike, but counting upgrades/farkles/gear that I bought because I was riding the IBR, it's probably a number between $5k and $6k.
I just rode from CA to MS a few days ago and premium gas was over $5/gal in some Western areas on I-10. It got a little cheaper as I moved East.