This has me interested. When you say sometimes they do it for a few bucks, will they tell you how much or is it at your discretion? I just don't want to rip someone off or rip myself off by leaving a huge tip. Also, would asking for a quick sketch of myself with Batman or Spider-Man or whoever, sound like a stupid request? Would they do quick sketches of me, or just the characters they are used to? Thanks.
Most artists will have a price list at their table. How much they charge may vary depending on what you want... i.e. a pencil sketch will be less than a pen and ink or full color piece. And it is up to the individual artist what they are comfortable doing, but I would imagine you would not have trouble finding someone who would do you with Batman or another character, though they might charge you a little more for it... having more than one character in the sketch is usually a little more expensive, but certainly worth it for something that is truly one of a kind and customized just for you.
In fact, asking an artist for a character that they don't normally draw may get you a better sketch than if you ask them for the same old character that they always draw...
When I was in line to get a sketch from Damion Scott, he was the artist on "Batgirl," and four people in a row in front of me asked for Batgirl sketches. He was obviously getting a little burned out on drawing her, and the sketch he did right before mine seemed a little rushed and he had not put as much effort into it.......
Well, I kind of wanted a Batgirl sketch, too.... but I didn't want to be the fifth Batgirl sketch in a row for the poor guy. So, on the spur of the moment, I asked if he could do Batgirl without the mask. You should have seen the smile on his face - he was relieved and excited that someone asked him for something different. He put more effort into my sketch and I got a sweet looking drawing of a maskless Batgirl.
Also, best sketch that my buddy got.... he asked Sean Chen for a drunk Tony Stark. Chen drew Stark holding an empty shotglass, bubbles over his head, with the caption "Cheers!"
So, the lesson here is, don't be afraid to ask for something unique! Odds are, the artist will be glad you asked for something different than the last ten guys who just asked for the character they are most known for drawing, giving them a chance to do something new.