Personally, I think the $1 per cap is much more onerous. For 4 caps, it's not so bad, but for the price of 40 you could buy a junk Model M on ebay and get a lot more for your money.
I have asked about extra Model M caps for sale, but I don't think many people want to part with them. I probably have a couple of incomplete sets laying around, but always seem to be way short on certain others (F2, *, the large numpad keys, for example).
Good luck with your search.