Seconding the suggestion that you post examples. Warning: long post ahead....I got a little carried away.
If you're talking about commission work that has an industry equivalent, jump down to the part in bold caps below. Unlike what some of the previous posters said, there is such a thing as a basic price.
I don't agree with Tale that both $15 and $150 may be fair, though. I personally believe that while it's fair to price as high as you like, there is a point at which you're pricing too low. $15 is too low for just about anything, unless it's something small that takes you an hour, and even then that's way below industry pricing. My personal belief is that if you don't have the skill to justify a fair price, you shouldn't be selling commissions.
But it's nigh impossible to give actual numbers without actually seeing examples ;;
The general advice is to price
at least minimum wage for your country. Personally I believe that you should charge
at least minimum wage for the country of whoever's currency you're using; this is for the sake of preventing the market from becoming too unbalanced due to outsourcing. It's already a problem in the corporate world, but it has an even more immediate impact in the world of the internet where you don't have to go through any effort to hire somebody from another country. (And this is especially true in the case of charging USD.)
I know, however, that some artists work slower than others, which can "artificially" inflate the price. Charging minimum wage is an ideal situation, but if you have the skill to justify commissions but can't work very fast at the same time, you have to compromise somewhere. In this case, you can adjust how you count your work hours (which has the effect in the end of adjusting your price to below minimum wage, AH WELL) which is different in everybody's case. You know your process best. I don't want to give you a specific number because it really depends on the individual and I'd really rather give personalized advice.
Generally, I recommend charging no less than half of minimum wage in your state with your adjusted rate. For example: If you spent 16 hours on a painted fullbody, but you spent 8 of those hours sketching because the design was difficult or you had to try out multiple poses and thus don't want to count your sketch hours, you would adjust your billable hours to 1 billed hour for 2 real hours. If minimum wage in your area is $7.25, your end price will be $58.
Blah blah that was probably a pretty convoluted way of putting it, but basically don't charge less than half of minimum wage even if you're a slow worker. Moreover, I suggest this as a starting rate; as you gain an audience and clientele, you should raise your prices. I'm not the person to ask for advice on how to gradually raise prices as that's an even more complicated question, but there are guides around the net. If you cannot build a base despite consistent work and outreach, then you probably don't have the skill to be selling your artwork in the first place.
Of course, even with all of this said, in the current market it's hard for most people to price fairly and still sell (especially regularly) except for those who are already popular. Above is my recommendation, but sometimes things are bad and you have to go lower just to feed yourself. I just ask that you push your prices below my recommendation only if you are truly in a emergency.
NOTE THAT THIS IS IN THE CASE OF PERSONAL COMMISSIONS! If you're working on something that has an industry equivalent (for example: providing assets for visual novels
),
there is a guide for industry pricing. You may or may not have heard of it before, but there is a handbook called The Graphic Artists Guild Handbook: Pricing & Ethical Guidelines. Freelancers cite this book pretty commonly; I've seen people refer to it for all sorts of art-related jobs. I linked you to their new digital format, but the print version is also available at the same site (that is the official website). You might have to do a little guesswork to match whatever you're doing to what's in the book since it is focused on the US industry, but I highly recommend it even for artists who don't depend completely upon freelance.
....I hope this long post was of some use to you, haha.