In my experience, as someone who repais o2ds/3ds, same price. In USD: A 3ds, newish, you can get for 140. Screens arnd 40 Motherboard around 80 Shell is 30ish. Just the basic compenents take you over sale price for a good condition one.

So I personally know it's more expensive to build. A lot more fun, a lot harder though

With what parts? It's not like a PC where you can pick and choose, and even if you found somehow every individual part, you'd had wasted enough time that it be more efficient to pay the higher upfront cost.

@Doomguy has a valid point. Just the sd slot will cost a bit. Also, you need small electronic soldering knowledge for several motherboard components if you are truly building it yourself