Basically, you seem to be proposing a system that allows an user to pay for posting a bounty with real money. Basically:
User A posts a bounty for N reps point, pays N * conversionFactor unicoins,
User B gets the bounty. He is awarded N reps point.
StackExchange gets N * conversionFactor unicoins
On the front, everyone is happy. You basically introduced micro-transactions in StackExchange business model, which would seem a good idea to finally help the network solving its financial issues...
Let's see what you have actually done: you have just implemented an official, approved way for anyone to buy rep for another user. Yep, that's right - bounties are awarded by the person who posts them (unless he forgets to).
So, under your assumption, someone can pay to give "free" reputation to any user he choices".
This is bad. Very bad. RegEx based HTML parsing level of bad.
Given enough time, this would go out of hand - now users can just "Pay-To-Win" to:
- get more rep points by using socks or voting rings to self-award bounties and thus more "visibility", by which I mean that inexperienced users are naturally inclined to trust more users with a lot of rep.... After all, isn't rep just "a rough measure of how much the community trusts you"?
- pay to unlock moderator level special powers on the site
- perform shady practices like "bounty selling" far more easily since the bounty rep has to come from nowhere
- even assuming good trust with no sock involved, "bounty gambling" (post a bounty on a post you own, hope the attracted upvotes exceed the bounty cost) suddenly becomes even more easy to do
- will undersell developer work even more, by offering a sort of "cheap outsourcing / do my homework" alternative to hiring and paying a developer what he really deserves. Thanks to PeterJ for pointing out this very important point I missed. If you need some work done fast, pay someone who put his life in that experience field what he deserves.
and so on...
Also consider that getting actual money in the picture would inevitably make handling system abuse far more problematic. Just think of a sockpupet / voting ring that also involves paid-with-real-money rep.... the Staff would have an hard time facing all the legal threats users will make when the bounty they paid for gets reversed because of a voting irregularity.
Sorry, this would only lend itself to problems... The system isn't just laid out to handle "paid help-desk services" and any attempt to steer it in that direction will probably just end up in a big mess.