Depending on the year, prices around the web are running from the low 900s to the high 1500s. One way to check is to see how much they are going for on ebay.
Depending on the condition, a Fender Dual Showman Reverb seems to go for between $950. 00 to $1200. Condition is very important. If yours is excellent, I would hold out for the higher price range.
