For me its totally dependant on the con. For example at Anime Express last year I think I totalled under $100 (before the dealer's room):
Transit:
$50 both dirrections. One tank down and back, plus half of one on saturday to cover side trips.
Food:
Aside from the traditional saturday sushi lunch, really we didnt' spend much at all.
Lodging:
Sitahtmai lived just down the road so we just stayed at his place at night. And even if worse came to worse we could just crash on the main anime room floor. Thank goodness they're down with freeloaders
Entrance fees:
$10 for 3 days
Dealer's Room:
Okay this I'll admit broke the bank.

After countless Master grade gundams and eva models, not to mention possible source footage (who could pass up 2 DVDs for $15.)
Emergency money:
We luckily had no real emergencies this year! No one even locked thier keys in the car **cough cough** Trevor **cough cough**
But at AWA, which is just a few miles north of me, I grossed almost twice that. Meals cost and arm and a leg in the mall, and registration was $25, still not bad for a con, but its interesting that its was cheeper for me to go to 500 miles Daytona than AWA.

<Orwell> WTF are they doing upstairs. Almost sounds like construction
<[Kristyrat]> Orwell, you see / <[Kristyrat]> when a man really likes a woman / <[Kristyrat]> or has a bottle of chloroform
<inthesto> He takes her stuff / <inthesto> And then poops in her vagina