If you use TurboTax Online, you can pay per return (a joint return counts as 2 returns).
However, if you are doing multiple returns, it may be cheaper to use TurboTax CD/Download (for Windows). You would be paying for a number of returns upfront, but the returns are discounted compared to the TurboTax Online returns. There is even a 20 return version of the software, which comes out to about $8 per return (before tax).
Please note that the Canada Revenue Agency (CRA) limits the number of returns that you can file online through NETFILE to 20 returns.