I've spent quite a bit of time looking into thsi as well.
The difficult/expensive part is finding teh right fabric at an affordable rate. For small things like a bikini top, you can kind of get away with basic cloth-backed vinyl scraps found at a local fabric store, for maybe $5-$10 a yard. The downside to thsi stuff is that it will stretch over time, and wil lrequire extra sewing aroudn grommets etc.
Thick canvas (like the Can-backs) is nice, but pricy by the yard, and you'll need several yards for a whole top. Plus it's not so easy to sew with, as mentioend above, need a good machine, strong needle, and patience galore.
I ocne stumbled across a bunch of fabric that is ideal, stashed in a scrap bin at a local store. They didn't know what i twas, got it for $5/yard. I've sicne learned it's essentially a PVC material w/ a woven pattern "look" stamped in it, and it has cloth embedded in the back. Very heavy, thick, and does not tear, crease, rip, or stretch. Problem, is, I cannot find it again - and everything close is - get ready - $15-20 a yard.
It would be ideal b/c the "fabric" is so strong, it dosn't require double-sewin gaeound edges/grommets etc, really cutting down on teh work... but enough of it would cost a small fortune.
I really like the idea of DYI top, but as long as the Kayline/STC top is <$600 total, it's hard to compete with, once you factor in teh cost of all the hardware and 40+ trips to the store to get this piece and that piece etc.