All convention centers and hotels would rent out individual rooms. Depending on the size of your party, they can wall off a hall and just give you a section. Expect to pay between $200 to several thousand dollars for a 12 to 24 hour block of time. The price would obviously go up for an entire weekend.
Hotels will usually give you a group rate for sleeping rooms (for out of towners) which they’d expect to be filled (or the conference/ballroom price might go up), and the event room’s price might be slashed if you order large amounts of food through their catering. If you rent our a room at a convention center or hotel, expect to pay extra ($80 to $1000) for an electricity or internet drop.
Unless you have sponsors, or are certain of a >150 turnout, I’d recommend against working with hotels or convention halls as a venue. This is especially true since most of you guys are new to this, and aren’t sure how big this event will draw. I’d hate for someone with grand aspirations to drop $5000 of their own money setting up for an event where only 40 locals show up. It would be cool for a player, but devastating for an organizer.
There are several community centers in Seattle, though I haven’t worked with one of those types of venues in 6 or 8 years. A single room (about the size of a school classroom) might run $100~200 for an 10 hour block. Electricity is usually not extra. However, community centers have more stringent hours, whenever the center is slated to close, they’ll kick your ass out. You’re usually expected to setup and clean up after yourself–usually within the time that you rented the room for. If the room is left messy, they might charge you extra.
Alternatively, there are local gaming clubs with brick and mortar venues. Metro Seattle Gamers comes to mind. They were formed by former UW students, mostly for tabletop gaming. These are the organizers for Dragonflight, the small gaming con that usually held at SeattleU or something. They used to have a house in Ballard (on Market Street), but I think they’ve recently moved locations to the foot of Queen Anne hill. I don’t know what their rental rates are these days.
Other than gaming clubs, there are still a few gaming stores. If anyone knows an owner of a gaming store, perhaps such a venue could be used. The good thing about this might be the rates. In exchange for publicity–or just out of a sense of good will–gaming stores might permit events to be held there free of charge. Space might be a concern, and managers might be leary thieves with so many people in their store at a time. I also anticipate that gaming enthusiasts of different sorts might not mesh well together. It would be funny to watch, though.
Most places will also allow you to rent equipment such as tables, chairs, TVs, power strips, and cords.