Hi Elcan,
First of all I should echo some of Rskman’s comments. I also think highly of the Tempflow mattresses and the designs and materials they use (see post #9 here) although they are a little out of your price range.
Memory foam does tend to sleep hotter than other types of foam although there has been some incremental improvement with some of the newer types of memory foam and gel memory foam that tent to be a little cooler.
Rskman also made a great point about temperature regulation in a mattress and how any of the materials you use in the upper layers can affect sleeping temperature. He also did some very specific testing for this. You can see more about the different factors that can affect sleeping temperature in post #2 here and more about the microclimate of a mattress in post #29 here.
The “moral” of all of this as Rskman mentioned is that even with a material like memory foam that tends to sleep warmer there are other changes that can be made that can reduce sleeping temperature.
Some of the better value online memory foam options I’m aware of are listed in post #12 here.
There may also be some good value available locally and if you let me know your city or zip I’d be happy to let you know about any of the better local options I’m aware of.
Phoenix