If I understand you correctly, you are asking about how much to charge for an app and naming the price at the point of sale (to moment you actually sell the app)?
It doesn't work that way.
Usually, you would meet with the client and get the requirements of "what" the client wants.
Once that is done, you then go home and put a quote together.
Then you meet again to discuss specifics (usually the requirements grow during the development process)
Then you ask for half the money up front and sign a contract.
Then it is time to actually develop the app.
At this stage, the client will ask for and get prototype builds to follow the development process.
When you are done, you show the finished app on your laptop (NOT on the internet)
You take the money (usually via pay pal because businesses pay by Visa)
Once that transaction clears, the customer gets the app.
One last thing.
If a customer doesn't have the budget for buying the source code/files, you can ask for less and only give away the app.
But depending on the contract, you might have to pass on everything as soon as you get payed.
So in closing. The price is negotiated once you know how much work will be required to complete the app.
Quoting is complex and no one will be able to help you because there are so many variables, it's impossible.
If it is a simple app, and you trust the person, you can just do it and get payed when you deliver ... but nothing in life is ever simple.