Hi Schurke1337,
Any multiple purchase discounts would vary from place to place and would depend on the policies and pricing of each manufacturer or retailer. The best way to know about any one in particular is to call and talk with them.
You can read my thoughts about bedinabox in post #2 here and I would tend to be very cautious with or avoid any mattress that had more than around an inch or so of lower quality/density materials in the comfort layers (such as 3 lb memory foam).
Dreamfoam is one of the members here which means I believe they are among the best quality/value available and blended Talalay latex is a very high quality and durable material in a comfort layer.
If you let me know your zip code or the city you live in I’d be happy to let you know of any better options or possibilities I’m aware of in your area. Hopefully you’ve also read post #1 here as well which has all the basic information, steps, and guidelines that can help you make the best possible choices.
Phoenix