A day pass for kumoricon is $30 the first two days and $25 on the last (cause theres not as much going on). Not counting the masquerade, there are usually two dances at Kumoricon. One is more a japanese/anime music style dance that usually takes place the first night. Then on the second night there is usually more techno/rave style music.
You do not have to be 18 to go to the dances. They usually start around evening time, it's just that they go into the AM hours, so minors can only stay during part of the time due to curfew laws. Last year there was an issue with setting up for the dance so the minors had to leave because it was after curfew before the dance even started, that's the only time minors havent been able to go.
There's ALOT more to do at a convention then be boredto death and glomped by 'younger kids' Though I dont know how old you are, at night there are MANY 18+ only events. There are some 21+ events as well.