This isn't complicated. Bruce can probably give you a quote (well, maybe not for the 680V!).
You get an STC, which involves making some changes to the altimeters and air data system (and involves installing a digital air data computer, if you don't have one already). I imagine if you already have a G950, the requirements are minimal, or maybe even included. If you have a factory original 1980 panel, you're going to spend a lot of money.