I think you have to do it yourself. I might be wrong but I've never noticed an "average" feature on TER.  Actually, you want two averages, since there are two separate ratings, appearance and performance.  

PM me if you need more help, unless someone comes along here with better information.

Try the Mathesar Report

http://theeroticreview.com/msgBoard/ViewMsgBody.asp?BoardID=1&Messageid=81883

Look under Escort reviews and search by average, its just down the line under the Review section.