Here’s a $49 mattress, but with delivery, comes to $80

https://www.amazon.com/PayLessHere-Inch-Memory-Foam-Mattress/dp/B0C3QPQ2DB/ref=sr_1_30_sspa?crid=21M4864OU5P3D&keywords=trifold%2Bmattress%2B6%2Binch&qid=1688222699&sprefix=trifold%2B%2Caps%2C896&sr=8-30-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9idGY&th=1

but you might want to consider a trifold mattress instead, that can be folded and stored away during the day, or double as extra seating. Also portable and can be used in a vehicle for rubber tramping, here’s a 4” one for $75:

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https://www.amazon.com/Olee-Sleep-Topper-Tri-Folding-Memory/dp/B075S5JTGW/ref=sr_1_7?content-id=amzn1.sym.1b55e175-aab2-4228-be7e-b2330901df8b%3Aamzn1.sym.1b55e175-aab2-4228-be7e-b2330901df8b&keywords=trifold+mattress+4+inch&pd_rd_r=46953ca8-f3b4-4e58-8c7c-38b9711cf3cb&pd_rd_w=CzcMz&pd_rd_wg=2v3kZ&pf_rd_p=1b55e175-aab2-4228-be7e-b2330901df8b&pf_rd_r=5VWYZ1CHF3E1GND59240&qid=1688223208&sr=8-7