Amazon.com has set 2504, Spinjitzu Dojo from the Ninjago series, for $40 today. The regular price from LEGO is $50. Amazon has offered the Dojo for this price a few times, so if you missed the deal just check it regularly. Chances are they will mark it down again.
My children own this set. It looks nice and has some interesting pieces, specifically the windows with Kanji lettering and the Sensei Wu minifigure. However, the dojo doesn’t stand up to their playing with it (it often requires reassembly).
$40 is a good target price for buying this set.