The first con you do, you probably won't recover all your loss up to this point, and that's normal. It takes a few conventions to start seeing returns on your investment in products and the work that went into them. Just try to have fun and learn a lot. Don't tell yourself you'll quit if you lose a bunch of money, because that is bound to happen. If not on your first con, then eventually. Everybody has a bad con once in a while, haha.
Pricing is divided into "base" and "margin." Base is how much that item costs you to make (and you can factor in labor if you spend significant time on each individual item) and margin is how much extra you add on when you sell it to someone else. Margin doesn't immediately equal profits, though. There are other business expenses you have to take out of your margin.
For an example, I bought 250 buttons this summer. It cost $75 for the bundle. The base cost per button is $0.30, so I rounded up to a whole dollar. My margin is $0.70. Once I sell all these buttons I'll make $250, which is a profit of $175. That's a margin of 70%, so it's a good deal for me. The problem is that I won't sell out of ALL my buttons at a single convention. I may sell only 50 buttons, which amounts to a $35 margin.
Costs like food, hotel, table fees, etc. also have to come out of that margin. That's why it's important not to JUST charge the base cost plus a little more. It's safe to go by 50% as an average. In other words, you should charge double what it costs you to make. If something cost $5 to make, charge at least $10 for it. It's not a strict rule, though. Like somebody said, selling products in dollar amounts ($1, $5, $10) is the easiest. That can mean rounding up or down. And as always, if the market can bear it, then by all means, you can increase your profits to a higher percent.
Different items seem to have a different face value with consumers, which I totally don't understand, but it happens. People see a full color poster I've done that I sell for $10 and they say "wow, that's cheap!" but when I try to sell my comic for $10, they think that is expensive. The poster takes about 10 hours to draw, and less than $2 to print each copy. The comic took about 500 hours to draw, and costs about $6 to print each copy. But, the poster is "a deal" while the book is "expensive." IDK, people are weird.
So lets say I sold so many products that I did $1000 sales, and I kept my average base cost down to 30%. That would mean I have $700 to cover all my expenses. if the trip costs an additional $400 between table fees, hotel, gas, food, etc. then I've only made about $300 in profit.
Important to realize that, because a lot of people get all that money and think I'M RICH!!!! and don't re-invest it wisely.