You can buy used GPU. It will be about 300MH for $4000 if you are lucky. Today it can bring you about $20 without electricity bill. I cann`t calculate correct because it depends on GPU model, but i think it will be about 1KWt.
So calculate it by yourself. And you have to test GPU yet or pay to specialist that can test it. And construct and configure rig.
I cannot say that this is passive income.
Seriously? Can you please tell me where you can get used RTX GPUs because I don't see one available on any online stores, I bet they aren't available anymore because the demands on GPU is pretty high still, some are even buying Laptops to mine right now