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My success with commissions seems to be varying by convention... at some I'll have so many I don't sleep, and at others I won't get more than one or two. (Last con, I got ONE commission for the weekend, and it was from someone who knows me and therefore was planning to get it that weekend.) So... it's hard to say whether you'll get them or not. Try to get your supplies as compact as you can and drag them along anyway, is my suggestion.

I think it would be worth it - I think your pricing would look very attractive to con goers as well as the art. Of course, it really does depend on the convention - ditto everything Vicemage said.

I think it's better to do a bunch of quick ones than a few expensive ones, or worse, a lot of expensive ones that will burn you out. So, for at-con conventions, I offer $4 sketches that are basically like your $3 sketches. Nothing fancier. To get more out of me, one would have to make a proper commission, and those cost more.

I think sketches are a great idea. There are a lot of people who want custom art, but don't have the funds for a full commission. Also quick stuff is great because they can take it home not much later that day, plus you're not tied down to doing one elaborate piece all day. I won't do any complex work at conventions, I just take folks' info to do a mailed piece later. Its hard to concentrate while working your table and you're under a lot of stress to finish it and do a good job while still managing to keep an eye out for anyone who comes near your table and stopping to talk to folks. Its just not worth the headache.