[Ethereum] How to make money developing DAPP on ethereum

The answer is you don't. Now I know you probably don't want to hear this but the HTTP JSON RPC was never meant to be used in such a way that it should be accessible from the outside. The RPC was brought in to existence so that you may develop against your own local node using your own browser.

What I suggest you do is that you setup a proxy that will proxy all requests to the locally running ethereum node instead of letting everyone directly interface with the ethereum node itself. This will give you controlled access and HTTPS.

The user could set the gasPrice to 0. Although this transaction would most likely not be picked up by the miners, the Dapp provider could mine all transaction going to his address.

This is an interesting question.
While we are still in PoW, the scenario you've described sounds possible.
I've made some calculations:

• It will take you an average of 800 days to find 1 Block if you are mining on a rig with one GTX 1080 Ti at 30MH/s .

Information given above is only for example. It can vary.

the Dapp provider could mine all transaction going to his address

It seems like you should estimate the load on your service and make some math. In conclusion you should decide if it is worthy to have such provider, working for 0$ profit or just to pay all these tx fees by yourself.