I have seen guys on boats go in together to buy queen size memory mattresses then rip them length wise to make two individual bunk mattresses on fishing boats. You are right about them costing hundreds of dollars. Then some fisherman learned that since boat bunks are usually longer than a twin and not always rectangular but narrower that they really were not perfect and still costly. Truckers have also dealt with the same issues as often some mattresses in sleepers are cheap & or offsize. Experiment with the differing thicknesses & dimensions on the website to investigate if acquiring new is affordable enough for you. Much of the site is geared towards retail customers who found themselves there by a google search & professional upholstery craftsmen & diy crafters. My guess is that it might be worth it.
I think buying a polyfoam/polyurethane foam that is pre measured to your exact specification (rectangle/trapezoid/triangle/whatever) would be best. By ordering the dimensions you can avoid paying for the overage that would come in the form of a traditional residential mattress. As it is not a complete mattress it is much cheaper. Instead of wasting money on extra material that will eventually get discarded, only pay for the length & width that you need. If it was me, I would invest in the added thickness while ordering an shaped piece that fits your application, perfectly. Free shipping over $75
The craigslist route is also fine as memory foam mattresses come with a cover that is zippered that holds the tags etc. These can be discarded to perfectly good & free polyfoam core that can be cut & used fro free. As many municipalities charge a fee for mattress disposal; it could be a win/win for you & they who are disposing of it. That being said. Some higher end memory foam mattresses have a secondary foam as a topper but just as comfy regardless of which side you flip. You may want to get a custom cover or custom sheets made to fit your application & or double as a seating area when not sleeping.