replacing HV battery vs converting to plug-in

The Prius in general (Plug In excepted) is not a good candidate for "PHEV" conversion. Here's why:
1) It is designed as a gasoline (primary) car with electric assist. The electric motors do not have enough power to safely propel the car absent help from the ICE. With electric mode only, the car is really underpowered and not safe to drive on the streets
2) The Prius has advanced, high-tech monitoring systems (called ECU's) that all work together to make this engineering marvel work. Adding more battery capacity requires modifying what one ECU sees which causes multiple problems with other ECU's.
The old adage that "every time you fix a problem you have to solve another" has major application here. A typical scenario may go like this:
Problem/Fix: The battery runs out or low in "EV" mode, I need a bigger battery (thousands of dollars)
New problem: Adding a bigger battery requires spoofing or/reprogramming the Battery ECU
Fix: Then I will reprogram/spoof the ECU so I can use the bigger battery (hundreds of dollars)
New problem: Now that I have this bigger battery and the ECU spoofed, the ICE still wants to come on when I accelerate normally
Fix: I can use OutOfGas mode to stop the ICE from coming on (cheap)
New problem: The stock electric motors/inverter setup are not strong enough to power the car in most situations and... if I want to use the ICE, I have to pull over, turn the car off and then back on to allow the ICE to be used (then you have the above problem of the ICE coming on too much)
Fix: I need to install a larger electric motor (thousands of dollars)
New problem: The inverter cannot power the new motor
Fix: I need a new motor controller/inverter (thousands of dollars)
New problem: I've spent so much money on converting my Prius that there is no return on investment and I can't sell it b/c noone wants a car that's had this many mods
Fix: I need to take every mod off and return the car to its original state (thousands of dollars)

There are kits available but, they are a dismal failure due to what I outlined above.

 

Restorations in general and conversions in particular are money losing hobbies. For what you'll need to drop to modify the Prius you'll be able to buy a PiP, a Volt, or a LEAF. The other downside is that YOU are the tech support & warranty. Used EVs have heavy depreciation and a used conversion pretty much has to be given away.

 

if you compare plug in cost to replacement battery, it depends on what you choose. a new battery at the dealer might be 3k, and a plug in might be 5-6k. but there are no trouble free plug ins, that's the bigger issue.