As far as non-free promotional items go, minicomics are a great idea! I'd probably charge a quarter for them, though, because if someone's willing to pay 10¢ for an item, they're likely willing to pay 25¢, and then you don't have to deal with as much small change.
I made a promotional comic back in the day, a 16-page b/w full-size comic that was an intro to some of my characters. It was professional printed, and I had... well, I can't say that I remember the exact numbers, but probably around 3000 copies made. They probably cost me around 40¢ each. I sold them for 99¢ (a regular 24-page comic sold for $2.50). I still have some left over that I bring to conventions, and even today they are my best seller, because a lot of people who may not be ready to drop $15 on a book are often willing to pay 99¢ to give it a try. Well, when I say it's my best seller I mean I sell more units of it than anything else, but even if I sell 25 copies I still make more profit selling 2 graphic novels. But I hook a lot of new readers with that 99¢ purchase.
(Oh, and when I sell it for 99¢, I have a small pile of pennies, and I offer one to people when they hand me a dollar. Most of the time they tell me to keep it.)