I'm afraid it has to do with "batch" sizes. For any part you have to pay the initial tooling costs, whether you manufacture 1 or 1 million. So, with very few TR2's, the batches of repro TR2 bumpers are small, and the tooling cost per unit is higher. For the TR3/4, there are more units made, so the tooling costs get diluted per unit.
Of course...it is also possible the supplier gouges for keeping slow moving parts on his shelf!?!