I just purchased Gary Young's R-380 listed on this board for $800. Depending upon condition and mileage they're seen often between $500-$1000. The 3.9 prices are all over the map primarily dependent upon mileage (and vintage). I think Pendy might have a handle on the engine market pricing.
"Umm, you can't get there from here, but if you go about 1/2 mile down the road..."