A lot of artists selling commissions are basing their prices on the amount of work instead of the time it'll take them. Portrait/waste up/full height, number of characters, the amount of coloring, the type of the background, additional difficulties like, say, detailed armor, etc etc etc.
I honestly prefer that approach myself, since time=value is a tricky concept. On one hand, what's stopping me from lying about the amount of time I spent on the piece? (Besides me being a decent person, of course, but we're talking hypotetically here).
There's the other side of that same coin, too. I work with texts for a living, writing them and translating them. The thing is, I'm a really quick writer. Not bragging - I won't estimate the quality of my own work here - but as far as speed goes, I'm fast. I think fast, I type fast, and if the work in question is interesting then I'll also be doing it in one go. If I measured the value of my work by time - I would be paid sheer cents.
In that regard I agree with Melon - when you're skilled at your job and you do it fast, then the time that counts here is not only the one you've spent on the piece itself, but also all the time that took you to get to this level.
And that's kinda hard to explain to some customers, which is why I prefer the workload method of calculating the cost. In terms of texts, it's usually about the amount of words or symbols. With art, well, see the beginning of this post.
I actually think that the concept of time spent adding to the value is sort of fair - if you spent N hours of your life doing something for a customer, you'd want to be paid something roughly equal to at least a paying wage for that amount of hours. Otherwise, in this economy, you're putting yourself at a disadvantage.
Now, the question of "Is anyone willing to pay you that amount of money for your work?" is a whole other matter entirely...
(P.S. I guess this falls back into the realm of "Should the worth of art be judged with capitalistic measures" which was brought up in another thread some time ago, and like... Oh, believe me, if I wasn't a hopeless optimist putting my art and self-expression above what's "profitable" or "sustainable", I wouldn't be perpetually broke