I don't know who said digital version of retail games should be less than $20, but THEY SHOULD cost less than the physical copy.
Physical copies of games cost money to make. The actual came cart itself for one, is basically a fucking memory card. Then theres the price of the printing and making of the game cases, as well of the costs to shipping them to different stores.
Wheras a digital copy of a game can be copied by it's produced as many time as it wants and can distribute it while saving all the costs that go into making a physical copy. So why the hell should digital games sell as much as retail?
Also with the digital copy... you can only trade it through 3DS transfer a maximum of 5 times, and if you don't have a club nintendo account when you buy digital, you'll NEVER get your club nintendo points.
While the physical copy can be played on any US system, so it can be resold or lent out, and you can choose when you use your club nintendo code, or sell it on ebay for $4-$7 or more based on how in demand that code is. I seen a fucking code go for $16 and !18 on ebay for the sake of nintendo's current promotion... thats nearly half the cost of the game itself.
So theres no reason what so ever that makes it right for Nintendo to sell a digital copy of it's game on the eshop for $39.99 plus tax, when the physical copy costs the same, and has so many more benefits.