Things are worth what they are worth, and $20 to $35 is what I expect to pay for a normal lens around f/2 in good shape from any major camera maker, though I might get one for less (and for some lenses, more). At any rate, the price of a normal lens in a long-abandoned mount of limited current popularity is not relevant to the price of a normal lens in a still-current mount of high popularity.
When new, a Nikkor f/2 was probably around 25% the price of a new Nikkormat, maybe a little less. So, to extend your logic regarding a camera body/normal lens cost ratio, a good used f/2 Nikkor should be around $2.50, which isn't realistic.
Besides, how in the world is it going to be worth it to convert a lens to Nikon mount, losing auto diaphragm function, when the most you can save is $35, and that's if the lens to be modified is free?