I can get a pretty decent memory foam mattress for under 300 with cal/eastern king sizes(whole seller's license). Just like car audio, you gotta stay away from the mainstream brands and get the good sh*t and keep your wallet fuller.
You need to know the specs too. 5lb density or 10 lb density, once you know finding your mattress will be a lot easier.
Foam Density
Foam density is measured in pounds per cubic foot (pcf) or in metric terms, kilograms per cubic meter (kg/m3). When a foam product is described as being a 5lb density product, what that actually means is that 1 cubic foot of foam material weights 5lbs. It is important to know the actual foam density to make sure it will provide you with the comfort and support you are looking for. Mostly the higher the foam density, the more support it will provide and the more durable it will be. Support is a very important aspect of foam bedding as it is defined as “to carry weight of, hold up, and keep from falling or sinking”. In bedding, this corresponds to making sure the product does not bottom out or compress to the point where it no longer holds the weight of the person using the topper, mattress or pillow. Foam density is independent of foam firmness (see ILD info below).
Figuring out the foam density of a specific pad is rather easy. All you need is the weight of the product and do some simple math. Say you are trying to confirm the specific density of a 2” thick queen sized topper. You take the dimensions of the topper and multiply them to get the total cubic inches of the product. In a queen that would be 80” x 60” x 2” thick = 9,600 cubic inches. Now we need to convert cubic inches into cubic ft by dividing by 1,728 (12x12x12). Our queen turns out to be 5.56 cubic feet of memory foam material (9,600/1,728=5.56). If the topper you purchased is 4lb density, your topper should weight approximately 22lbs (5.56x4=22.24lbs). In most foam manufacturing there is a standard variance allowance of up to 10%. Some retailers and marketers are not stating their foam density ratings. Don't be surprised when you get the product. Make sure that they state that information up front and guarantee the product you receive to be of the specified density rating.
IFD (Previously called ILD-Indentation Load Deflection)
The IFD (Indentation Force Deflection) rating of the foam will determine the feel of the product. Most memory foams fall within the 10-14 ILD rating, while latex will have a much wider firmness range (16-40 IFD). The higher the IFD measurement the firmer the foam will feel. IFD is taken by measuring the force in pounds required to compress (indent) a sample of foam a specified percentage of its thickness across a surface area of 50 square inches. This is normally done on a 4 inch thick piece of foam. Foams manufactured in the United States are typically measured at 25% indentation or at a depth of 1” with a 4” foam sample. Firmness of the foam measured by IFD is an indicator of the surface feel of the foam, while support (density) is an indicator of the foams ability to support the load placed upon it.