Hi Wendy1107,
Just in case you haven’t read it yet … the first place I would start is the tutorial post here which has all the basic information, steps, and guidelines from the most important forum posts all in one place. It will help you know what to look for and what to avoid and perhaps most importantly the steps to take to find the best possible mattress for you.
You are in a price range that there aren’t a lot of options for an all latex mattress although there are a few specially priced mattresses such as the Roma, some synthetic latex mattresses, or some latex hybrids that would be in your budget range. You can see some comments here about the Roma which is a great quality/value choice and some of the other lower budget latex or latex hybrid mattresses are linked in posts #3 and #4 here as well.
The first step though would be to do some local testing to see how you feel about and on a latex mattress and to get a better sense of the general firmness levels that you prefer. Of course when you can’t test mattress in person then it’s always a good idea to make sure you have a more detailed conversation with the manufacturer who can give you some guidance about their mattresses as well (see mattress firmness/comfort levels in post #2 here).
If you let me know your city or zip code I’d be happy to let you know of any options that may be available to you locally so that you can at least have a chance to test some latex mattresses.
Phoenix