Well, the three basic colour plates solid red #18, Solid purple #19 and solid blue #20 have been this price for a while direct from Nintendo and its a fair price if your not into flashy plates. Looking around earlier release plate stock is starting to dry up a little, and this price is cheaper by enough of a margin to warrant a post i thought.
+ a lot of new3ds owners so they might not be aware you can get some decent plates cheap.
I've recently bought the #36 green solid colour set and they look great. These are well worth £5.99 each only problem is min order is £20 for free P+P so best to get all three and a cheap goomba (or other villain, well saying that goombas are ok. for stomping that is!:)) keyring of your choice to put you over the £20.
Heck the way plates are being discontinued you could keep your fave and sell on the other two at a later date.