Setting up gasPrice in testrpc doesn't influence the transaction cost

Question

Testing this function:

function withdrawAll () 
public
stopInEmergency () {
    if (!msg.sender.send(12 finney)) {
        throw;
    }
}

My truffle test:

it("Should let correct payouts and withdrawals", function(done) {
var my_contract = My15.deployed();
var unit = "finney";
const user_6_initialBalance = web3.eth.getBalance(user_6);
var  user_6_gas_cost = 0;

return my_contract.withdrawAll ({from: user_6});
    .then(function(tx_id){

        user_6_gas_cost = web3.eth.getTransactionReceipt(tx_id).gasUsed * web3.eth.gasPrice;

        const user_6_finalBalance = web3.eth.getBalance(user_6); 
        const withdrawn = web3.toWei(12, unit);
        const recieved = user_6_finalBalance - user_6_initialBalance;
        const diff = withdrawn - recieved;

        console.log('gasPrice: ', web3.eth.gasPrice.toString(10)); 
        console.log('withdrawn: ', web3.fromWei(withdrawn, unit));
        console.log('recieved: ', web3.fromWei(recieved, unit));
        console.log("gas cost: ", web3.fromWei(user_6_gas_cost, unit));
        console.log('diff: ', web3.fromWei(diff, unit));

    }).then(function(){
        done();
    }).catch(done);

});

When running testrpc --g 1

gasPrice:  1
withdrawn:  12
recieved:  9.127600000008192
gas comission:  0.000000000028724
diff:  2.872399999991808

When running testrpc --g 100000000000

gasPrice:  100000000000
withdrawn:  12
recieved:  9.127600000008192
gas comission:  2.8724
diff:  2.872399999991808

Why doesn't gas price influence the amount user_6 receives?

Answer 1

In your example you are using the network gas price and you should use the transaction gas price.

user_6_gas_cost = web3.eth.getTransactionReceipt(tx_id).gasUsed *
    web3.eth.getTransaction(tx_id).gasPrice;

If you do not set the gasPrice in a transaction it will use a default value (with testrpc appears to be 100 Gwei). It will not use the network gas price (returned by web3.eth.gasPrice). You have to explicitely set the gas price you want.

my_contract.withdrawAll ({ from: user_6, gasPrice: web3.eth.gasPrice })