Amazon offers the Zinus 12" Memory Foam King Green Tea Mattress for $280 with free shipping. That's the lowest price we could find by $80, although we saw it for a buck less in February. (It's also $9 under what Amazon charges for a queen.) It features 3" of memory foam, 2" of comfort foam, 3.5" of high density foam, and 3.5" of base support foam.
Amazon offers the Zinus 12" Memory Foam Queen Mattress for $227.45 with free shipping. That's $62 off and the lowest price we could find. It features a 1" microfiber cover and is infused with green tea, charcoal, and natural plant oil.