It depends on your engine.
If your prop flange already supports a C/S prop, you can just add a prop and a governor. Vans has great deals on props, so you could probably do it under $10k. If your engine is not designed for a C/S prop, you have two options:
a) Swap engines
b) Get an MT or Sensenich Electric prop.
The MTs are not typically as fast in cruise as a hydraulic Hartzell, although the difference is noticed more in something like a Lancair than an RV.
On the plus side, the MT is typically much lighter than the hydraulic solution, perhaps even as light as your fixed prop (which would eliminate recalculating all of your CG stuff). I don't have much information on the Sensenich, but you can get them through
Eggenfellner.
Anyway, assuming an MT you will probably be in the $12-13 range for prop and controller, depending on the difference between the dollar and Euro. The Eggenfellner is $9 or $10k (3 or 4 blade) for the prop and another $1k for the controller.