What is the minimum amount of investment to really make money on Aave and Compound protocols if you take into account gas fees?

Below 10k bucks, it is not even worth it.

If you want to invest less than that into DeFi, the Binance Smart Chain will allow you to do it without eating all you rewards into gas fees. 

A transaction on BSC will cost you 10 cents instead of 50 bucks on Ethereum network.

Furthermore, you will find the same kind of protocols allowing to lend coins and borrow some other ones. Aave and Compound have been replaced by Fortube and Venus and their tokens FOR and XVS have a huge potential.

Venus passed a new milestone yesterday by launching a governance based on XVS. However, the token value has been affected by recent rumors of hacking through CAN token. As CAN has been retreated, the issue is fixed, and XVS is set for growth.

That's not a personal advice, just a feeling. Do your own research, and enjoy DeFi on BSC.