Well, prices vary according to the event - if you were to come to Yaoicon, it'd be $60 for the full three-day weekend. I can't tell you what the prices at other conventions in the US are like, because I don't go to many! (What with living in the UK and all

)
Hotel stuff, I generally let other people worry about XD If you need a room, there's usually lots of people on here that have space and who're happy to take others in with them - I've done that every time so far, and not had a problem yet! (And met some
awesome people in the process <3) The convention itself is held in the hotel, so you don't have to worry much about getting lost between them or anything like that ^_^
Usually, at a convention, there'll be a whole load of 'panel' events going on ('How To Draw Manga-style' 'Fanfiction Writing 101', things like that) that you can just turn up to, plus various anime screenings. There's also usually various cosplay-related events, such as a masquerade (showing off your outfit on the catwalk), omake (people performing skits while in cosplay), a cosplay competition and usually a ball/dance of some sort. In addition to these, Yaoicon has events such as Bishounen Bingo, the Bishie Auction, Pictionary, Cosplay Cafes and so forth.
As well as the panel events, you can expect to see literally hundreds of people in cosplay (if not over 1000), and you will usually end up making a few new friends while you're there too. That's where the true awesomeness of a convention lies; the attendees. As a rule of thumb, everyone tends to be very friendly and open to chatting with you if they're not rushing off somewhere; I know that for me, most of my time is spent milling round talking to people, and it's brilliant ^_^
And~ as if that doesn't whet your whistle, there's also a dealer's room, where you can pick up all kinds of merchandise, usually at cut prices, and sometimes before the official release!
It's very hard to really describe what to expect at a convention; that narrative above doesn't give any clue as to how much fun it usually is. Just imagine 1300-1500 excitable/happy anime fans in one building, and you'll perhaps get some idea of what
really goes on XD