Supply and Demand makes prices go up and down; it's the basis of the economy. Old classic games are known as classics for a reason so demand is almost always present for these kinds of games. Especially with the new wave of old school-style games on the market as well as other throwback games like Smash Bros., I believe demand has only increased for the people who missed out on these games back in the day.
I think that the most expensive Nintendo game would probably be Earthbound. It costs $70-80 just for the cart and $230 if you also want the guide and the box XP But it's soooooo worth the price of admission.