Green roof drainage plane
Even though it's only one square, four grand does not sound unreasonable for a membrane roof and slip sheet that someone is going to grow plants on top of... If I have to provide any kind of warranty, that is.
The system we use is prevegetated and uses lightweight grow medium in a 1x2 tray system. Even with the lightweight aggregate the structure still has to support a lot of extra dead load when it's wet/snowy (140lbs/sf IIRC). Never heard of the wiffle ball method.
We use EPDM as our drainage plane, but my honest advice is call a pro. As noted above some contractors may be able to install the whole system for that price.