Cost of a new Prius: more than $20k.
Number of small front wheel drive cars in the US which could potentially be retrofitted in this manner: tens of millions.
Price of a retrofit people might buy (especially as gas keeps going up): $1000 - $2000.
Potential market for retrofits at that price: many tens of billions of dollars.
The average person wouldn't be able to install these on their own. Do you have any idea how dangerous the electrical system in a hybrid is?!?!? They kill people. You have to be certified to work on the damn things. You think every garage in the country is gonna get the training needed to install one of these "retrofit" things. Nope. Buy a prius, get a bike.