That's a fair point.... and I'm sure with the support and following the game has, it wouldn't be too difficult to hit a target. 3D modeling, and graphics in general, always take an abundance of time.
I'm not sure if Red uses it or not but a lot of game devs that use Unity for the more realistic looking games use something called SpeedTree and then you can buy 3D models optimized for it. Even something like a 3D model for a cattail costs 20 dollars and hiring a 3D artist to make the same thing would cost even more for a one off.
That can really add up and wish there was a way other than just buying more copies of the game to help out with that, but Red still being able to maintain the quality, etc. so personally I think a donation thing would be the best solution.