Just to be fair and upfront, I know from looking into vtubers specifically that the model itself easily is $150 usd to 500. So your statement of payment for the artists worth needs to be more clear.
If you're wanting to do JUST PNGTubers, which is at the least 2 images with slight changes between them, that comes down the quality and style of art as per the cost / artist's whim. Those do not need animating unless you're doing a GIF PNGtuber style which is uncommon currently. Because of the wild differences in cost, these PNGTubers can cost as little as $80. Again, your statement on the amount of money the artist will get per commission needs to be clearer.
You also should likely state upfront your plan. "You give me the art, I can animate it" ect ect. To which I will extend another piece of knowledge, the animation if done well (termed rigging), can cost $500 usd to 1000.
Even on a conservative side, you saying the artist will get $8-20 usd is embarrassingly low and will make anyone question your promise of animation skills. Now, turned around, are you asking for an artist to provide you with a model that you can then practice animating for a vtuber? That's different, and actually there are free models available for use with the animating tools Live2D Studio