Shutterfinger covered the basics re: RB Graflex, mirror, etc.
Pricewise: Expect somewhere between $600 and $1600 for a 4x5 RB Super D, depending on condition, what's included, functionality, etc. Even really beat ones seem to fetch at least $500-600 these days. Mint is usually more like $1200-1500. Series-D or other series would be less $, but also older and potentially lesser condition. The main thing of concern in these is the shutter. Everything else is relatively easy to work on/replace/fabricate/etc.
A nice AE is usually $350-500, again condition matters a lot. Pentacs run from about $250 to 500, generally.
The Jolo lens board is the lens board to use for these, but I am not sure if they are still available or not, if so they are about $150-200. But fabricating a lens board is not too difficult, so maybe that's a wash price-wise.
Cutting down or replacing the mirror and it's frame so it clears the AE: that's some custom work, and neither trivial nor cheap probably. The camera internals need some modifying to make that work. Finding a new mirror is easy enough, firstsurfacemirror.com is where I got a replacement one for one of my SuperDs (just a stock replacement one). But shortening the mirror frame, adjusting things: that's all a fair bit of work. People here have done it, but it's not something that's real easy to explain, you have to sort of dig into it and figure it out as you go. Doing so also compromises the stock light-trap design of the RB, but depending on how you use it, shouldn't have a huge effect, since the focal plane shutter still prevents light from hitting the film. (The mirror acts as a light trap too in stock form, but not in shortened form.)
Pics of my SuperD projects (one 4x5 stock, one 4x5 modified, one 3x4 converted to 4x5):
http://edsawyer.com/lens/graflex_superD/
http://edsawyer.com/lens/graflex_superD_pentac_woody/
http://edsawyer.com/lens/graflex_superD_3x4/
Glad the CY/Eos adapter is working out well for you too!
-Ed