Does anyone have a rough idea of the cost of getting a C extension installed (non mechanical).
Does anyone here have a D extension?

It's in the neighborhood of a thousand bucks, depending on the wood, whether the luthier cuts into the scroll or fits around it, etc. A D extension will cost somewhat less as there's less material involved and the extension won't interfere with the scroll.

You should get one with a capo at the E so you'll still be able to play Donald Erb pieces.

-dh

Yep, about a grand. KC Strings has one that is worth checking out at www.kcstrings.com