Good question, but I would think a look at the tax rate for small businesses that have incorporated might be close.
It might be costly but it might be worth the freedom it purchased.
Maybe taxes would lead to leaner churches, more focused on Christ and fellowship, rather than spreading out, building, acquiring property and larger estates, diversifying, and so on.
The simpler and more compact the building and property, and the more focused the purpose, would reduce the tax burden of the congregation.
Interesting idea.