Except with all the safety equipment, crumple zones, airbags, sensors, etc. I would expect an increase in price.

Some of those $10k cars in the 90s had more plastic in the bodies than cars today, e.g. Saturn S-series, where all body panels below the belt-line were plastic.

It isn't necessarily the cost savings one might expect though, because steel panels can also be load bearing and part of the crash structure, which is not really practical with plastic panels.