You can estimate the price by doing a dummy booking for a WTP return, which can be done online as a multi-city.
When doing a UuA you will also pay any extra taxes/fees/charges for a CW ticket over the underlying WTP ticket, which will be quite minor.
Best to book over the phone all at once, or book an open jaw WTP online and call to upgrade. Two one-ways will be far more expensive as you will be booking a more expensive one way WTP fare.