A lot of people don't understand the expenses that Artist Alley vendors face. I've had tables that have cost as much as $400. I had to pay to have all my comics printed, for my display equipment, and for hotel rooms and food and travel expenses. On top of that, there's the labor costs that you should theoretically be paying yourself for working 8-12 hours straight. If you total it all up, the costs can seem astronomical and extremely hard to reach with a table full of $5 comics. That's why so many tablers choose to ignore all other expenses besides the most tangible one: the cost of the table. Reaching that goal feels attainable and keeps up morale.
On the other hand, making table is not a viable target in the long term, and covering ALL expenses eventually needs to become the real goal. I've set up spreadsheets to help keep track of all my expenses, and once I've officially made it into the black, I consider the con successful. That said, I still pay attention to the moment I make table, because the rush of accomplishment that comes from crossing that threshold really gives me a boost.
Rock on, guys, and never stop drawing.