One says that the cartoonist should charge for sketches at conventions. Period. You’re there to make money, after all, and this is an important source of revenue. And, truly, most of your fans are only too happy to pay a small fee to get a personalized piece of original art.
In the other camp are those who insist that free sketches are an easy way to thank your fans for their support. Moreover, asking for money for these sketches is an insult to a fan who already paid to get in the doors to see you at a convention.
So, who’s right? Both are. Prominently post a price list on your table that includes sketch prices (including a free option). You should offer price breakdowns — in other words, you’ll do a foreground sketch for $XX or a full scene (foreground-plus-background) for $YY.
If you haven’t done this before, you may be surprised how many people are eager to buy sketches. Many convention-goers will bring books for you to sketch in, but some won’t so be sure to bring along some good-quality illustration board.
Meanwhile, practice a handful of simple character sketches until you can execute in under a minute each. These are the sketches you’ll offer to anyone who asks for a free sketch. You might even consider these sketches — or somewhat more developed versions thereof — for inside the books you sell at the booth. But you have to be able to do these free sketches quickly.
Any amount of time you spend with your head down at the table is time that you’re not doing what you came to do — interacting with fans, selling merchandise and promoting the comic to new readers. You can’t afford to get too wrapped up in these sketches.
Done right, this system can be used quite deftly to help ensure a profitable convention. Often, a free sketch can be used to drive a sale. For example, you can offer a free sketch to a person who is waveringly considering buying one of your books. And you can slow down your tempo on a free sketch and encourage a potential customer to leaf through one of your books. If you think s/he might possibly buy a book, you can facilitate some quality time with your merchandise as s/he wait for you to complete the sketch.