Like bargain wine and crotchety magazine editors, plastic gets touchy with age. Sunlight is the biggest enemy, but even tucked in a climatecontroller hanger, the compounds that give plastic its flexibility outgas, leaving the remaining material prone to cracking or shattering from a stress it would have sloughed off 10 years prior. Everything aviation costs roughly an order of magnitude higher than an analogous non-aviation part, and plastic interior pieces are no different. The small redeeming point with interior plastic is that parts for the majority of GA aircraft can still be had, and that you can save a bit by doing the installation yourself, if youre so inclined. 
OEM and Otherwise
Bobby Cook, Parts Manager for Cessna Aircraft Parts told us, “We don’t sell a ton of plastic. We used to sell more, but these days everyone is tightening up their dollar.” Its logical that non-essential items like a crack in the left seatback would be one of the first things to get hacked from the aircraft repair budget. Cook admitted, however, that this drop in sales may also be from more business going to suppliers of non-OEM parts, looking for a better price.
The price delta between OEM and non-OEM sources is huge-commonly over 50 percent. Parts resellers like Cessna Aircraft Parts buy from the OEMs. The advantage is the parts are usually a perfect fit and are at least as good as the original.
We say at least because any newly manufactured part must meet the current structure and burn standards. Plastic technology has improved in the past 30 years. That said, formulations for UV protection and fire retardancy are somewhat at cross purposes, so the new part may not outlast the original if its an antique.
All plastic parts have to be cleaned up after casting and Cook says some aftermarket parts fall short here. “Some [buyers] have come back to us and bought