There's two ways to determine a reasonable price.
First search on eBay. In the filter click on sold items and completed items. Then search through the items noting the prices of original and complete. You can even reorganise with highest or lowest price first.
Second, buy Jared Cope's Star Wars Tracker app. You get a month's trial free when you sign up, and somewhere on this forum is a thread with a code to obtain a monthly discount for SWFUK users.