Cheapest way to get a 3ds?

You have a couple of choices. There is the option to buy a second hand 3DS, which I think can be bought for less than 100 dollars, at least where I am from. Your other choice is waiting for a holiday such as Christmas or Black Friday, where you'll be able to get a discount.
 
i'd go for ebay. even a used one at gamestop is $100.
or if you have any local used cd stores (like here we have a small one called Last Stop CD Shop), go there. i got my DS there for $90.
i'd say unless u get lucky, on ebay the lowest ur gonna get one is $70.