how you gonna support the last one with 200% ? out of your own pocket ?

We can cover it with fees.
You're taking 2.5% of the deposits.
If the last person sends 1BTC, you need to get 1 additional BTC from the fees. That means you need to have a 40 BTC volume. There are not much ponzi's with that amount of deposits.
Plus, you won't have any profit with that volume; you need an even larger volume.
I wish you good luck, but I don't think you'll be able to cover it.
The last person sends 1BTC, and I need extra 1 btc. (We earned only 0.5btc(2,5%*20BTC))