The problem with coming up with a proper price is that time isn't the only factor. Quality and service both factor in. Both of which are subjective. In fact, all kinda are.

Time it takes is subjective, because what might take a long time for an artist could be done quickly by another.Short of charging per hour, using time as a metric is... faulty.

Quality: This is what most people look at for an upfront cost commission, but even this is subjective. How do you rank things on realism vs stylization? At what point does cost increase? Can it decrease?

Service: How good of a service do you provide to our customers. Are they satisfied? Are you able to and or willing to work with them on their small issues?

All these factor into prices.

As for your prices specifically: Yah, you could probably bump it up a bit.
You have gone from
http://www.furaffinity.net/view/18543449/
to
http://www.furaffinity.net/view/20076242/
In about 6 months. This is a sufficient quality improvement to warrant an increase in cost for an upfront cost commission :P

As for how much, that gets... tricky, and is a bit much to discuss in a single comment without making a wall'o'text

Honestly depends on how many commissions you get. As much of a good idea it is to charge what you feel you're worth, there's a point where prices because reasonable and then some people don't think so. Really is up to experimentation to find that line. I think the prices are alright as they are, but I've never bought work from you, so I can't honestly attest to their fairness, so my words come with a grain of salt.

I've been undercharging for years and ... it sucks ...

It boils down to supply:demand really. If you get more commissions then you can handle with slots filling up in record speed then its a definite time to up the prices a little, but if you don't get that many commissions and find it difficult to get work then a decrease in price...luckily its obvious you wont have to consider the latter! ^w^

Personally judging by your prices for say Full-body,full shaded i wouldn't hesitate to commission you for a image (one day i will. i got a bee character i would love to see rendered in your style <w<). which to me is a sign of maybe a bit underpriced. i would easilly pay 25 for what you charge 20 for (according to your current commission info).

I was going to up the base cost of everything by $2.50, which... well, isn't much higher but still XD