With a family of 4 (2 adults, 2 kids), we spent a total of $600, not including badges, which were pre-paid.
We stayed at a Holiday Inn which was about a 10 minute drive from the convention center. For 5 days, it cost us $325. (Got there on Wednesday and didn't leave till Sunday.) We drove there on one tank of gas.
Near the hotel, there was a Target superstore, so we stocked up on fruit, sodas, juice boxes, and sandwich fixings for 4 main days of gaming, costing a total of $85. The hotel room fridge was very efficient at nearly freezing the soda cans and juice boxes just shy of the exploding stage, so when packed, they kept our sandwiches nice and cold for lunches. Breakfast consisted of the daily complimentary hot breakfast. With the exception of the D&D party (dinner courtesy of the Ram), dinner was fast food from the nearby joints.
The rest of the unaccounted for cashola was spent on parking, event tickets, the obligatory dragon picture, and product.
The only thing I'm changing for next year's GenCon is to actually stay at one of the hotels connected to the convention center. It's hard work dragging a bunch of swag and product around to and from the car.
Some odds and ends tips:
-Don't buy the convention food. GenCon has no control of their prices. Instead, walk about a block from the convention center to the center circle mall and grab some grubbage from the food court there. Or, like we did, stock pile food from a grocery store and store them in your hotel room/hotel fridge. Also, try to find a hotel that offers a complimentary hot breakfast. Cheap eats.
-Unless you find a great deal, you are better off getting a dealer's business card and ordering online of after the con, especially if it's a bulky item.
-The anime rooms are a great place to grab a seat and take a break between gaming sessions, not to mention spacious and less hectic than the rest of the con areas.
-GenCon this past year was very very good about having water stations scattered about. Take advantage of this great perk, courtesy of the GenCon staff. No reason to be dehydrated.
-Before taking a picture of someone in costume, please please please ask. Almost everyone in costume will say yes and even pose for you. Nothing worse than turning around and getting blinded by a flash in the face. (Yes, happens a lot and this is based off experience.)