+1 on rock crusher's answer, well said.
Dirt bikes certainly go through a lot of engineering but the product lifecycle cost analysis for motos is a completely different equation:
a) for r&d, tooling, supply chain costs, they are sharing these costs (in things like engines, brakes and suspension) across multiple lines of bikes (cruiser/sport/dirt/4 Wheeler/snow mobile)
b) the r&d cost, and the initial tooling investment especially was definitely paid for by earlier higher end bikes a few years prior and is now a sunk cost, allowing them to churn out older generation tech for only the supply chain, materials, mfctr, and marketing cost. Early in the lifecycle of a new technology, high end product, r&d and tooling is 30-60% of the cost of the unit, depending on how the company decided to capitalize the investment in their books. Capitalization is an accounting method where the different costs of the product, though incurred up front, are distributed across a longer period of time in the books, usually a 3-5 year period, corresponding to the useful life of the product. Once all outstanding onetime r&d/tooling costs are completely 'depreciated' from the books, the incremental cost of the product (from an accounting perspective) is materials + manufacturing/assembly + marketing +supply chain and distribution, which, 5 yrs later, may only be 40% of the original cost to make it in its first year, hence the price being ~40% relative to the company's higher end model on the same year.