I don't buy all that reasoning. The engineering is already done and the machinery is in place to manufacture all the parts neseccary for a complete tractor assembly. The only additional expense to make extra parts should be the raw material, energy used, and very little labor to complete the manufacturing process. Shipping & handling is usually passed on to the purchaser as an added cost to the buyer.
While it may be not quite as simple as that, it still isn't as costly as you presented it. I'm just referring to manufacturing, not dealership expense.