Hi Nowhisky,
Welcome to the Mattress Forum! 
I’m sorry you didn’t find us before making your purchase, but at least you’re here now! B)
I would include Kingsdown in the “major manufacturers” group that I would completely avoid (along with any mattress where you aren’t able to find out what is inside it). They don’t disclose the materials in their mattresses and they are not very good quality/value. I would tend to avoid the Kingsdown unless there was a compelling reason to do otherwise.
The major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
Regarding your question about pricing, I would tell you to completely ignore the MSRP and MMAP pricing, as these are generally just starting points used to inflate the perception of the “deal” you’re getting. Here’s a video that gives a behind-the-scene look form a retailer’s perspective about such pricing. Even a MMAP price can often be three times the actual cost of the mattress, depending upon brand.
If possible, I’d recommend that you “rethink” your mattress buying process and start again. Start here with our Mattress Shopping Tutorial. It’s a lot to read but by using this information you’ll be in the situation to make a much better choice.
Let us know what you decide to do!
Phoenix