If a contract is payable and requires some amount of Ether is it possible to take the gas cost out of the sent value instead of directly from the user. Although both these values are coming from the user I would want a user to only be charged the value they are trying to send.
Lets just say for example a lottery contract. Each user would pay, for the sake of the example, 1 Ether to enter the lottery. However I would want to take off of that 1 Ether the gas cost, adding only the difference to the lottery contract and having my user pay the exact amount they wished to pay.