I would rewrite the code as the following to avoid all unnecessary conversions that can induce errors:

    const gasPrice = await ethers.getDefaultProvider().getGasPrice();
    const gasUnits =
        await new Contract(
            address,
            abi,
            ethers.getDefaultProvider()
        ).estimateGas.functionName(params);

    const transactionFee = gasPrice.mul(gasUnits);
    console.log("transactionFee in wei: " + transactionFee.toString());
    console.log("transactionFee in ether: " + ethers.utils.formatUnits(transactionFee, "ether"));