Hi chip,
In the original post I linked … there is a list of the members here that sell online who I recommend that each in their own way I believe are among the best quality and value in the country. Some of these have mattresses that are in the price range you are looking at.
If you are looking more specifically for memory foam … then there is a list of some of the better options I’m aware of in post #12 here (with some overlap because some of them are members included in the first list) that also have some mattresses in the price range you are looking at.
Other than that … there are probably hundreds of online choices but as you have hopefully read, the ability to assess their relative quality and value depends on their willingness to disclose the specifics of the materials inside them so you can make meaningful “apples to apples” quality and value comparisons with other similar mattresses and of course on their knowledge and experience that can help you choose a mattress that is suitable for your body type, sleeping positions, and preferences. No matter what the quality or value of a mattress … if it is not suitable for your needs and preferences or you are dealing with an online merchant who will sell you “anything that they can” … then it would have little value to you. With an online purchase … the ability to return a mattress or exchange layers if you need to do any fine tuning to adjust how it feels and performs for you may also be an important part of your personal value equation.
If you have read the mattress shopping guidelines in the “read me” post … you would see that I wouldn’t recommend visiting either of these places and most of the time you spend there would be wasted time if you are looking for quality and value in any budget range.
Post #3 here and the link it includes to the adjustable bed thread may be helpful.
Phoenix