example of `getGasUsed` helper function

getGasUsed.js

// helper function (you can create your own `helpers.js` file and import to your tests)
const getGasUsed = async (tx) => {
  const { gasUsed, effectiveGasPrice } = await tx.wait();
  return gasUsed.mul(effectiveGasPrice);
};

// your tests
describe('how to calculate and consider gas costs', () => {
  const oneEth = ethers.utils.parseEther('1');
  // deposit `oneEth`
  // then...

  it('should withdraw contract funds', async () => {
    const initialOwnerBalance = await owner.getBalance();
    const withdrawTx = await yourContract.connect(owner).withdrawFunds();
    const gasUsed = await getGasUsed(withdrawTx);
    const finalOwnerBalance = await owner.getBalance();
    expect(finalOwnerBalance).to.equal(initialOwnerBalance.add(oneEth).sub(gasUsed));
  });
});