No rules, no baselines, charge what you think is fair.
Well, one rule. TAKE AT LEAST HALF PAY UP FRONT. You might feel strange doing this, or like your clients will feel like you don't trust them- and you don't. You have to assume they won't want to pay. It sucks, but if you're super nice you do work for free and end up with useless crap you can't display (because they'll get it for free that way) and you don't want or need in your portfolio.
Usually people work it out as time + materials, but you have to tweak that based on current trends and what you can get. You can spend a thousand hours on something but that doesn't mean you can get $10,000 or whatever for it.
And it works the other way around, too. As you get better and can work faster, you change that around. If you can kick out some really good lineart in half an hour, you don't charge $5 plus materials, you charge what you think it's worth.
The better you are, the less time it will take, so the more you have to charge per hour if you're calculating it that way, which is why many people will charge by the job for certain things.
What you can reasonably expect people to pay is very, very subjective. It depends on who the people are and how well known you are.
If you go into a gallery, you can see two paintings of pretty much the same level, size, media and subject, and one is $25 and the other is $250. What's the difference? The artist who can charge $250 CAN charge that much, because people will BUY it.
I think people generally start off doing $10-20 for a sketch, with each level on difficulty adding fees, and of course the background and other characters and stuff.
Start out charging the least you feel you can afford, and raise your price as you get people interested.