The hotel is the highest cost that I've found. I tend to plan on having a roommate. This year it'll be my good friend, mystery writer Carole Nelson Douglas. How did she get to be my good friend? By my holding my breath and agreeing to sign up for being roommates with a total stranger.
Some people even have 3 or even 4 roommates with two double beds. I probably couldn't go that far. Too many people to share a bathroom with. But Carole absolutely brings food, and usually brings a cooler to pack with ice that she can then visit a grocery store and fill. In-room picnics can be fun.
There are also morning parties where breakfast or at least food is served that's free. You'll have to stand in lines, though. And while I was in New Orleans, I found an Arby's next door where I ate a lot of my meals. In another city, it was a Denny's next door and an IHOP at another. They always have some cheap meals.
If you're pubbed, the publisher often takes all of their authors out one night for a big dinner. And if you're agented, your agent will usually take you out once too (if they attend).
But I tend to save up and enjoy myself. Meals are times to catch up with friends I haven't seen since the last RT. But I do wind up pretty tired. There's a ton of walking and often up and down stairs because the elevators slow to a crawl.