It is, it really is!
And what would I pay? I'd pay whatever you ask, to be quite honest, haha.
However, the internet at large can be fickle (and everything is too expensive for the internet), so I reckon yeah, between $4.99 - $9.99 USD would be the way to go.
I'd just make sure that you make it clear in an Early Access trailer what the money would be going towards. More art assets means more gameplay variety, so I'd try to make a point of that, for sure!