How much you pay for using TurboTax Online depends on the specific product you are using. See here for current price information.
If your tax return shows a refund, you can choose to pay for TurboTax from the refund but there is an additional charge of $40 and your refund must be large enough to cover the cost. See this help article for more information.
You can also pay for TurboTax when you are ready to file; TurboTax will ask for payment in the File section. You won't be able to print the return until you pay.