Prices can be affected by a number of things.

The S10 has a fixed pitch prop and normally aspirated Limbach. It may not have folding wings, which detracts a lot from convenience and speed of rigging - making it a one man, easy operation that take just minutes.

A S10-VT has a turbo charged, intercooled engine and prop that has take off and cruise positions.

There are also two panel shapes, the earlier standard panel and the "mountain soaring" panel that is a little smaller, but easier for the pilot or passenger to see out the opposite side of the canopy - since it's introduction, I think most Stemmes are sold with this option.

I think those are the main options and those that would be the most expensive to retrofit.

A few options, at least back when I purchased my ship in '97, like long range tanks, if you ordered you paid for them, and if you didn't order them, you got them for free anyway, as "all" the wings came with them!