Indeed, CX will recalculate what you flew (or if you hadn't flown yet the cost of the ow ticket) and refund the difference between those two minus the cancellation fee. It's not so bad that you actually get a refund at all as often the balance may be negative as OW flights tend to be so much more expensive than returns.

An alternative, if you have more trips planned, would be to change the return part of your ticket to a date when you can use it in which case you'd be charged the change fee plus any difference in fare which is likely to be far less punitive unless you breach some of the original ticket conditions.