I'd agree to this, with some caveats. First off, no-one should recieve money for code that is not in production. No advances. While this is a bit of a harsh rule, that way we actually know exactly what we're getting, and that we're getting it.

Beyond that, though, there's the rather difficult questions of "what deserves payment", and "how much payment"... and I'm not sure how to answer those, in the general case, or in any specific case.

The best thing to do may be to, when making a feature request, say "I'd be willing to pay $N to the implementor of such a thing", or indeed, to reply to other people's requests with that. Then it's a matter between you and whoever implements it.

For that matter, when people announce their patches on PMD (something all us pmdevils should do a better job of, myself not the least), offer payment-in-thanks.