You wouldn't (or, rather, shouldn't) find a traditional artist like Picasso (as an exaggerated example) and ask them to paint something for you without compensation, right? Same thing should apply to this when it comes to original work.
That doesn't mean that someone won't volunteer their services or work, but it should not be expected that all work should be free (unless you're making content for your own project/own team's project I guess!).
As for pricing, I have three thoughts:
a) Should be up to the artist being commissioned to set their price - if the person doing the commissioning doesn't like the rate, well, that's for them to decide.
b) I understand not everyone is rich and could afford dropping, say, $200 on a single commission (for an animation or something), but please remember to keep in mind the amount of labor and time that would go into a specific work - and this applies to both sides of the agreement. Artists shouldn't undercut/lowball themselves for the service they're offering and commissioners shouldn't be angered if the rate quoted is high, especially if it might be high quality, time-heavy work.
I don't really have a dog in this race, since I do happen to volunteer my work/services or post what I make as resources as I'm just a hobbyist - I do this for fun in my spare time on days/weeks after work or on my days off that I feel like it, and when I do offer to contribute to something, I do it with the assumption that it will be done on my own time with no deadlines beyond my own self-imposed ones, if I have any. However, if you're commissioning something and expect a level of quality or completion by a certain time, you should be expecting to act like you're actually hiring that person and offering compensation for the service being rendered.
One other side note in response to 7743's post, regarding "it is better to make it free and share it with everyone" - while freely-usable resources are great, there are very much situations where this should not be the case. Having everything be free to use means that uniqueness and individuality can suffer in projects.
Let's say you wanted to commission an animation for your main character's promoted class that's "custom" on the tier of being non-generic in appearance, with a legendary weapon in tow, smooth frames and effects, etc. Something that's so tailored specifically to your project and your character(s) should, frankly, remain that way. Making it available to everyone no longer makes it a special feature that sets your project apart. Sure, other things can still set your project apart (gameplay experience, maps, story, etc.), but just because something is made does not mean that it should be appropriated to everyone's toolbox.