The next convention on my horizon is Loncon in August, and considering that I live pretty close, it shouldn't be too big an expense. Still, between a £65 ticket (which, for five days, is admittedly great value) plus train fare of probably about £15/day, it's going to come to about £150 before I factor in food, drink and swag. Admittedly, that has nothing on PAX - I do want to go to PAX someday, but I'd rather not think about the price of flights from the UK - but when you're freelancing and not getting a regular income, it's still a lot of money.
Conventions are so much fun, and I love them, but they're a big expense even if you're close by.