Hi yubbers,
Welcome to the Mattress Forum! … and I’m glad you found us 
Yes you’re right that it’s much more difficult to find good quality mattresses that sell for under $500 (in queen size). There is more about different budget ranges in this article.
You can see some comments about this mattress in this topic and there are some red flags here that would make me cautious and I would follow the suggestions in post #6 in the topic (checking the law tag and checking with the manufacturer) before considering it for a purchase.
Phoenix