Electric batteries can function as lead-acid batteries or they can be more advanced through Nickel-Cadmium or even Lithium-Ion depending on the particular technology that will be used in creating the car.
Some of the elements used in creating these batteries are expensive (platinum being a good example as it is used in several PHEV and EV designs these days) and that expense will need to be passed onto the consumer regardless of whether mass production takes place or not. Mass production can reduce the overall cost of putting the cars together, but it cannot reduce the cost of rare materials and there are many of those rare materials present in PHEV automobiles.
Going beyond the realm of the physical and focusing more on the specifics of the human contribution to this equation, PHEVs have not had the same amount of research put into them as their internal combustion counterparts. When you look at a PHEV today, what you are really looking at is genuine research for 5-10 years behind that car, some research for 10-20 years before that and sporadic research for 60-80 years before that. Contrast that with the gasoline vehicle which has had genuine research for nearly a century and it becomes easy to see the disparity in technical finesse between the two. This is partially what contributes to the two previous problems discussed above, but it has consequences for just about the entire field of the comparison that can be made.
There are many other problems with PHEVs that are more minor in nature, but the major ones have been discussed above. Some of these problems can be reduced with further research and some of them can be solved with research that will hopefully come up with substitutes that can be used in a cheaper way. However, these problems only serve to illustrate the main point that PHEVs and EVs may be the way of the future, but right now they are a new technology that still has a long way to go in order to achieve market parity.