How much 'extra' as in price?? That's kind of difficult to answer since pricing can vary based on the model and dealer location, etc.
If I remember correctly, the fiberglass/hardwall is/was an optional feature on the Springdale & "should" (logically) increase trade-in value too.
You might try the NADA site, using a previous year's model...
http://www.nadaguides.com/RVs
should give you a basic idea on re-sale price difference between the two exteriors. As an added note, the fiberglass is typically a bit heavier
(weight) than the aluminum siding if all other construction is the same.
Hope this helps.
Terri, the Chevy co-pilot