That could be entire whole other blog, which would entail a lot of advice that I , quite frankly, don't feel like giving out for free.
but I'll give you the high points of the high points.

you can estimate the value of your man hours. For example, when I do commissions I charge 40-50 an hour. That's the price of a top notch electrician...I'd be a top notch illustrator, my time and skill is no less valuable. If you don't feel like a high tier artist, at the very very least...you are as good at art as the guy making hamburgers at McDonalds is at making hamburgers, if he's making 10.00 an hour...and the piece you made took you four hours. 40.00 is a ballpark figure to start with. Shave off a little if you want if it's an edition that has multiples ( 10 or whatever) but don't fucking divide that number by the amount of copies, that buyer is not getting one tenth of a piece of art, is he? He's getting a piece of art.

OR/also You can compare the quality of your work to that of others, see what people get who are comparable in skill and quality get...then add 10% because you're probably seeing all the flaws in your work and none of the flaws in the other guys. This is somewhat flawed because that other guy might have several years worth of following built up...and sometimes an idea supersedes skill. SouthPark is no great masterpiece of animation, it's well below average...but the concept/idea/charm is what makes it a sensation as opposed to the 50 other cartoons that have come and gone but were drawn better. Not the greatest example but you know what I mean. But if you find enough works that are comparable to yours and figure the average price, you'll have a good starting point.

BUT with both of these I'd give you some baseball advice "swing hard in case you hit it". Meaning don't be shy, error on the side of asking for more, don't feel like you are out of line for asking for it. You are doing something they cannot, and unlike getting an electrician...they can't just pick a different one, your work is unique to you. Someone likes your work but the price is too high, chances are they'll ask if you'll take less if you are easy to contact, then it's up to you. Usually my answer is "nope" but I've been at this a good long time and know what my work/time is worth, (unless the person is really good at feeding my ego, then I might have a weak moment, but that's rare)...the newer you are though, makes it a tougher call.

Charging more as you get better? yes...I suppose that is true but more so than that, the longer you are at it, the better of an understanding you have of what you can ask for based on experience and past purchases.

There's other angles and aspects, but I'm not a sales instructor and not all of it is universal AND I'm not a slave to my benevolence, hahaha.

That should help though...yes?