A similar thing happens with me as well. Let me suggest you a few pointers :
- Charge the client for the initial cost , no matter how low it is. Think till the point where you think it is affordable. Do not go below that point.
- The partnership terms should be profitable for you . Therefore, an ideal percentage would be in the range of 40-50%
If you do not ask for 40-50%, you will eventually be ending up with a loss on that project since the income (proposed income) of that project would need to fill that void of money which was created when you did the project at a lower price.
I once had an offer to develop a web app where the client told me that I should develop the web app for $300 , but the actual market price for that was $800.
I was promised a lot of things but I suspected the client would not live up to his promise.
You cannot trust a stranger completely in such matters.
Hence, it is very important to validate the client in terms of money, attitude and their past business transactions with other developers . With this, you will get an idea of what person you are interacting with.
If the client does not agree to these terms, I suggest you not to accept those projects at all.