Amazon offers a variety of mattresses at different price points. Multiple product pages and different depth options make comparing prices tricky, but you can expect to pay around $300 for a 10-inch thick queen size (the minimum thickness we'd recommend for an average-weight adult).
Mattress sales are highly competitive in general, but Amazon isn't the best place to find them, because the ticket prices are so low to begin with. There are sometimes see further discounts on the best Amazon mattresses, but no real pattern to when they'll arise. If you are determined to find a discount, Prime Day is perhaps your best hope (more on that in the next section), or the Black Friday mattress deals and following Cyber Monday mattress deals in November.