hehe, i need to check this thread more often. was thinking about getting it going again.
as for commissions, the pricing is really up to you. if you're doing it for a living you need to base your rates on your cost of living, cost of materials, how much profit you want to make, and your time, of course. if you're doing it for fun with a little extra cash i think the factors to balance are:
1) how long it'll take,
2) how much you feel your time is worth,
3) how much it feels like fun vs. how much it feels like work and,
4) very generally, your skill level vs. current market value
the first is pretty easy. start with the levels of detail you're willing to do: linework, full color, single character illustration, multi-char with background, or whatever, and roughly estimate how many hours it'll take - leaning on the generous side. i think it's also a good idea to give free quotes with a brief description of what the client wants, just in case it doesn't fit exactly into your predetermined guides.
the second is a little harder. is the time you spend on it time you would be otherwise spending at a job? if so, how much would you potentially be paid there? if not, what kind of pay could you get with your skills and level of experience?
this relates to the third quite a bit. if it feels like fun and it's something you'd be doing anyways, then i think you can be a little more lax in your rates. on the other hand, if you're drawing something you really don't like and it's causing you stress and frustration, then you need to make it worth your while with payment.
lastly, you can check out other illustrators available for commission and their prices, and roughly gage their skill level and yours. i wouldn't make this a huge consideration if it means really bringing down your prices because you don't feel your stuff is up to par with another's, but it's worth taking a look at and taking into some consideration.
so with all that in mind, figure out a good rate for yourself - hopefully somewhere above what you'd be making at a menial job but still within market value for your skill level - and multiply it by the number of hours required to get your pricing, then maybe tweak those values a bit so they make good sense to you and to a potential client.
keep in mind that paypal does charge a fee for those transactions. it's free to convert an existing paypal account into a business account there so i still think it's a good way to do secure online transactions. i'm not completely sure what the formula is, but i think there's a base fee of a couple dollars, plus a small percentage (maybe 1-2%) of the total amount.
i must admit i don't much at all about advertising and exposure - maybe sporkii can help you out there =) we spent a good chunk of time coming up with the guidelines and pricing for her commission page here:
http://signalburst.org/mariska/commission/anyway, that's my two cents about it. hope that helps a bit!