There is just too much going on with the Prius, in terms of computer control to know what propane will do. Propane has less energy per unit than gasoline when used as an engine fuel. Don't ask me how much because I don't know. However propane does make a good motor vehicle fuel when you design the fuel management system to change fuel metering and fuel/air mixture to more propane friendly values.

I don't want to say that a propane conversion of the Prius cannot be done, however I'm convinced that it would require Toyota's intervention to have the software updated to handle the very different qualities of that fuel. However, the Prius really isn't a good candidate for a propane conversion because the vehicle only has an 800 lb load limit and adding a pressure tank would take up a lot of space and would also significantly encroach on that load limit reducing the number of people you could carry or the amount of luggage you could carry.

Consider that the Prius achievers using regular gasoline what even a propane powered vehicle may not and that is an AT-PZEV emissions rating.

The value issue with propane converted vehicles had more to do with the manufacturer and their engine manufacturing process than it had to do with the fuel. GM was notorious for not using hardened valves back in the day. So, if you were buying GM vehicles and wanted to do a propane conversion on them you had to special order the vehicle as a "propane ready" vehicle, which meant that GM installed hardened valves in the engines of those vehicles.

Ford on the other hand has always used hardened valves, so any Ford product could be converted to propane with no issues associated with valve wear or deterioration.

It has been said:
Hybrid drivers come in 3 flavors, greenie, techie and cheapie. Pick any 2.
2005 Prius, Melinium Silver over gray, package 5 (AI)