I have owned and sold two late model S3 trucks………both clean, minimal rust, lots of upgrades, etc. for around $12K - $14K.
Both 88" trucks with hard tops (not that I think the top really matters on a series truck).
As John mentions above, price will depend on the quality of the conversion and the condition of the truck. Both of mine were original 2.25 trucks and were unmolested.
84' is a cool year as it's the last year of the Series trucks. Sure it's a soft dash vs. all metal, but it's a nice truck and the end of an era.
Here is one of the trucks I sold for comparison to what you are buying:
------ Follow up post added June 18th, 2015 09:43 PM ------
Here is the other one: