My normally aspirated non pressurized IV was converted to a turbine about 2 years after I bought it.  The conversion took about a year and cost about 250K all in...engine, prop, belly tank, baggage compartment tank and filler on fuselage, air conditioning, cowl modifications, engine instruments, throttle quadrant and paint.  It would be much more expensive today...I think.  Assuming you use a Walther the engine and Avia prop are reasonably priced.  I believe I paid around 50K.  

I flew it for another 2 years after the conversion.  It was the most awesome airplane I have ever flown.  They will fly 300 KTAS even in the teens.  My only issue was the fuel consumption.  They will use 40 GPH.  If I could afford 40 GPH I would have no other airplane for cross country travel.  Truly awesome!  If your mission is more just to fly than to fly cross country, that 40 GPH might dissuade you from jumping in on a Sunday afternoon to shoot approaches and do some low level sight seeing unless you own a fuel farm.

If you do decide to move forward with a conversion I strongly suggest you contact Lanny Rundell at Southern Air in Winnsboro LA.  He has done this conversion before, is extremely knowledgable and honest as the day is long.

Berni