Without a doubt fibreglass is a useful material for quick and relatively cheap repro parts – but it is not in any way as durable as plastic. Your Montecarlo and Scorpion cars are all over 30 years old and yet most of the plastic has survived. So our policy is to try to source good second-hand parts, examine these and repair where necessary, and then re-market these to you all. a good example of this are the vent grills behind the rear window, bonnet and bumper grills and and the external rear light panel.
The cost for tooling for extruded plastic products really is massive so I can appreciate why costs for these are high where offered.