score:10
It's called an open-jaw flight/ticket, and you usually have to deal with it and buy two one-way tickets. Most bookings sites will allow multi-city options.
For example, on Matrix Airfare Search, you can choose the multi-city option. Then you can specify which date ranges you want to take the A-B and C-A option.
Sometimes it's cheaper, sometimes it's more expensive, sometimes people use hidden-ticket options or drop the return leg, but there are risks involved in these tricks (See other questions on this site for those topics).