Hi Goosney,
It certainly could be a good buy depending on the quality/density of the polyfoam and most importantly all the objective, subjective, and intangible parts of your personal value equation that are most important to you. Of course it would also depend on what you are comparing it to and on the other mattresses you are considering because “value” is relative to the mattresses you are comparing and the criteria that are most important to each person … not least of which is how well a mattress matches your needs and preferences in terms of PPP.
I would tend to avoid buying a mattress with the specific intent of adding a topper afterwards unless you can test the mattress/topper combination in person for PPP. It adds an additional layer of uncertainty because you have to first choose a mattress with no weak links and then buy a topper that you can’t test in person on your mattress. A topper will feel and perform differently depending on the mattress that is underneath it so choosing a topper that you can’t test first can be almost as difficult as choosing a mattress in the first place.
A mattress/topper combination can be a good option if you can test them both together in person or if you make an initial purchase that is less than ideal for you and it needs some extra softness and fine tuning but your odds of success are higher if you are able to test what you will end up sleeping on in person. Of course if you can test the mattress/topper combination ahead of time then a mattress topper combination can be a good choice because a topper is like having an exchangeable comfort layer that you can replace without having to replace the entire mattress if it softens or breaks down faster than the base mattress (which is likely) or if your needs and preferences change over time. It can also add to the durability of the mattress under it because the topper will take up most of the compression forces that come from sleeping on the mattress.
Phoenix