Snap presses are great!
I don't have one myself but am lucky enough to have some lovely friends who let me use theirs

I'm sure that it worked out to be around the 20-25 nappies made mark that it begins to break even if you compare the cost of using the steel snaps you can buy from s/l. The best part about using the resin snaps it that there is no risk from rust either, though the one I have made with steel snaps didn't rust.
It costs about $5 for enough steel snaps for a cover/nappy and $100ish for the snap press and snaps (can't check ATM cos snapsaustralia won't load for mefor some reason).

So if you're going to make a few then the economics would work out better in the long run.