How often ( if ever) do Nintendo made games, ( Mario Kart for eg) go on sale on eShop?
Almost never, and the reduction is low. On the Wii U, the games that sold badly got IIRC about 25% reduction at some points, or a discount for a bit more using My Nintendo coins.
The biggest price reduction on Nintendo's own titles happens when they do the "Nintendo Selects" reissue of a game. Then the whole title become budget priced (both retail and eShop). But I think that's only happens at the end of a console's life.