The lithiums should be somewhat smaller and about half the weight of the nimh pack we now use. Looks to me like the increase in price for a plug-in hybrid car, would be the newer lithium traction batteries and the charger that would accompany them.

 

A lithium cell is about 1/2 the size and weight of a NiMH cell of the same capacity. If you mean non-plug in for "the nimh pack we now use," note that a PHEV will need about 10x the total battery capacity of a non-plug in to get a minimally acceptable EV range at decent speeds. That will require about 5x the size and weight of battery in a lithium chemistry rather than the 10x size and weight of a NiMH chemistry.

Lithium's are not half the cell volume per capacity, but could be nearly half the total battery volume due to more efficient packaging (flat cells rather than round can cells). However, there is a lot of other factors, such as cooling requirements that will have an impact. Half the weight is a good rule of thumb for these cells.

-- Alan