Hi pamela,
While the retail prices they are using for comparison are not “real” (I doubt anyone ever paid this) … the prices you are looking at are significantly less than retail.
If these prices were for a new mattress … they would be remarkable value but of course they are for a floor model which has been used so the real question is more about how you feel about a slightly used mattress. In other words … its value to “you”. You can also check to see how old the mattress is and how long it has been on the floor (the manufacturing date should be on the law tag).
As you mentioned … there would be no warranty (you can see the PLB warranty here which clearly states that there is no warranty on a floor model) but warranties are are usually more of a marketing tool anyway. Warranties cover defects and these will usually show up early in the life of a mattress. The main reason a mattresses needs to be replaced is the gradual loss of comfort and support which isn’t covered by a warranty because it isn’t a defect so warranties have little connection with the useful life of a mattress. The biggest assurance you have about the quality and durability of a mattress is knowing the materials inside it … not the warranty. Latex is among the most durable of all foam materials although softer foam of any type will be less durable than firmer foam of the same type.
Besides the value of a mattress though, its suitability for your body type, sleeping positions, and preferences are just as important. This means that you need to test a mattress carefully for pressure relief and support in all your sleeping positions. This is especially important if your situation is somewhat challenging (such as having back problems). With a mattress that is that soft, there should be no issues with pressure relief but I would make sure that you are in good alignment. This would be especially important with the very soft additional topper (which is even softer than the comfort layers of the mattress). You can read more about testing for support/alignment in post #11 here. Your much lighter weight increases the odds that softer thicker comfort layers will “work” for your alignment but personal testing is the most accurate way to know for sure. Keep in mind that you can always soften up a mattress that is too firm but it’s very difficult to make a mattress firmer that either starts off or has become too soft.
If you are confident that either the mattress by itself or with the topper provides good support /alignment, then it would depend on how you feel about a floor model and the amount of “discount” you would assign to its value to you. Each person may have different feelings about buying a floor model. For some it would be great value. For others it would be “no way at any cost”. This is a personal preference decision but if you are comfortable with its condition and the esthetics of buying a slightly used mattress … then the “value” based on the materials is good. Again … my only caution is that no matter what the quality or value of any mattress … if it doesn’t provide you with the pressure relief and support/alignment you need … then it would have little value to you (even if it would be great for someone else).
Stores that encourage or even allow negotiation are usually more of a marketing gimmick. You can read more about this in post #6 here, along with post #2 here, and this video (by someone I respect highly) is also well worth watching to see how mattress pricing and fake sales usually works.
Some stores have “flat” pricing where their best price is the one listed and at best they may throw in something extra (such as a free protector, delivery, pillow etc). This is usually a sign of a better retailer or manufacturer. Stores that play the “negotiation game” are really only trying to give their customers a false sense that they “got a deal” when in fact they didn’t. Having said that … it’s always a good idea to ask.
The most effective way to negotiate would be to find a mattress either locally or online that has similar materials in a similar quantity (such as comparing 12" of Talalay latex to other mattresses that contain 12" of Talalay latex) and a similar ticking (cover) and quilting layer and using these other mattresses as a reference point. Of course this would depend on knowing the materials in the mattress you are considering (which is the case of the Beautiful you do). I would then show the local retailer the other similar mattresses that were available to me (locally or online) and ask how close they could come with the one they were selling. If you are comparing a mattress that is being sold locally to one that is available online … then I would be willing to pay a “premium” to some degree to offset the additional “risk” of an online purchase that you can’t test ahead of time (a good guideline here would be 20% - 25% but this would depend on each person’s own “value equation” and risk tolerance). If you do this you are making a much more meaningful and “real” “apples to apples” value comparison based on materials and real value which would be much better and simpler than trying to figure out how much to reduce what may be an inflated selling price that only “looks good” when it’s compared to a fake listed or “regular” price.
This of course wouldn’t really apply to your circumstances because you are looking at a floor model which is already less than most if not all new 12" Talalay latex mattresses that were available either locally or online.
It certainly never hurts to ask though (and I certainly would) for something extra or to make an offer or ask “is this the best you can do”. The worst that can happen is that they say yes (it’s the best they can do) or no (they can’t add any extra value to the price they have offered).
So the bottom line is that if it appears to be in good condition and has no flaws (except the one you mentioned with the topper) and you are comfortable with a “used” mattress, then it would seem to me to be good value.
Hope this helps … and I hope you let us know what you decide 
Phoenix