Hi The Riddler,
I have a listing for those that have a more restricted budget in post #4 here and the posts it links to also include many of the better lower budget online options I’m aware of as well. I know this certainly isn’t a complete list, as there are hundreds of mattress brands across the country, and sometimes when you are on a tight budget you can find some good values from local mattress factories as well. One of the “less expensive” innerspring mattresses using higher quality materials would be along the lines of the Eco Sleep from Arizona Premium, but that starts at $895.00.
Phoenix