In-app purchase custom price

As noted by other poster, you cannot have variable in-app purchases. You have to select one of the provided price tiers.

Now assuming you are making a taxi-service app then what you need to do in your app is provide the user with a list of prices that are fetched from your server for example how much will it cost to go from point A to point B. You can then "charge" them outside of the in-app purchases using your own pricing service (credit card over the phone etc).

Again in-app purchase doesn't work with what you need.