Transaction cost in a truffle test

Question

I've a contract under test and the method I'm checking is supposed to transfer an amount, and in order to check I need to have the cost of the transaction. I'm using this code:

var tx = await instance.withdraw(web3.toWei(25,"finney"));
  console.log(tx);
  let ownerBalanceAfter = await web3.eth.getBalance(accounts[0]);
assert.equal(ownerBalanceAfter-ownerBalance,web3.toWei(25,"finney")-web3.eth.gasPrice.mul(tx.receipt.gasUsed)); //withdraw done

The test fails with some unexpected small difference, even if I apparently try to compute the gas cost. Internally the method call transfer(address), that I know it consumes other gas, but shouldn't this be counted in the tx.receipt.gasUsed? So what is wrong in my balance computation?

Answer 1

I found that I can specify the GasPrice for the transaction and this ensure me on what is used, so I just changed:

var tx = await instance.withdraw(web3.toWei(25,"finney"),{gasPrice:web3.eth.gasPrice});

adding gasPrice as argument, and test now works.