There has GOT to be a less expensive way, what have all you other sewing-disadvantaged handy people done?
We bought our foam at Ikea, went in and tried the different m<span style="font-size:10pt;line-height:100%">attresses</span>, found one we liked within the thickness we wanted (needed), cut the mattress using an electric kitchen knife (learned about that here). We bought the double size, and used extra from width and glued it on to the end to make it long enoungh. It is meant to be a mattress to sleep on, we find it better than our home mattress. Ikea just had them on sale for up to 50% off, not sure if they still are.
We also bought our own fabric (local discount shop), read sewing sites on the internet, talked to people at the fabric store, the local foam store (who charged $90/cushion) to get tips about how to sew cushions, and another good tip was from the upholstery place where I bought the zipper, they also had good tips.
If you go this route, one of the best tips we got was to make your foam 1" longer and wider than the cushion, then when you put the foam in, in expands the fabric out nice and tight.
If you have the original fabric and cushions all around, it is easy to pull them apart and use as templates to cut new (and to see how it all should be put together). We had bought a trailer where the rear foam was replaced, but not cut to proper size, so we had to figure out all our measurments on our own, I had to sew, put in place, find out a little too big, cut the foam, cut the fabric and re-sew a couple of times, it was worth it though, they turned out fantastic.
Oh, price wise, around $200 fabric/zipper/thread, and around $200 for foam, front and back.