Here's what I would charge, if I was at your level of talent.

Base prices
I think Sketches should cost $5-7
Colored Sketches should be $10
Flat Color (No shading, additional lighting etc) $15-20
Full Color could be $35-40
Reference Sheets should be $20-25 (Based on what I paid for one.)

Fees:

Extra Character(s) $3 a character, $% or more depending on the complexity of the character
Adult content: $10 - $15 (Sketches & colored sketches) & (Just an additional $5 - $10 on Full color images and reference sheets.)
MLP characters (I don't really see the point in charging MLP characters separately, as in their own category. Unless you can do them a lot quicker than other art or they take longer.)

My fee's are remaining un changed, I charge extra for MLP because I personally hate everything about MLP thus do not enjoy drawing it. Hence the fee, if I'm being paid more to do it then fine but if someone doesn't want to pay my fee then there are several artists that will.

My adult fee will stay the same too since I am not fond of drawing porn, the fee is cheaper in my stream commission sessions.

Colored sketches have shading in them so no way I'd go that low for them.

Ref sheets while they might look easy, they are not. You have to pay extra close attention to the character in question and make sure the anatomy is correct.
They may be easier to some artists but not for me.

Thank you for your input, I'm waiting on more replies to get a ball park idea on what to charge.

Perhaps you could offer variants on the ref sheet to save time? Like greyscale versions and color versions? Not sure if that would help you though.

Um...reference sheets take up a LOT of time for an artist. At her own current prices she's not even making minimum wage. $20-25 for a ref sheet is a huge, HUGE loss on the artist's side then.

Hey, I'm not a professional artist. I'm not surviving on the income from my art, so I don't exactly know what prices are needed. I was just asked what I think some prices should be. And I know ref sheets aren't easy, I had to wait months for mine, though I think that was down to the artists own busy schedule. Art itself isn't easy, it takes time and patience, I understand this. My pricing is simply based on what I would charge and what some other artists roughly charge. Admittedly some of my price recommendations are a little low.

For real? I should start charging more too then ;_;

I know it might sound strange, but offering close friends and frequent customers discounts DOES help. Plus, offering like $5 off or something for people referred from friends or other clients, while cheaper then originally, will increase over-all sales. Yeah, each pic might be less pricey, but you chance getting so many more jobs. :)