FWIW, this is also one way of seeing what Google Summer of Code is. Sure, the GSoC students have mentors from within the MetaBrainz community, but they're getting paid by a 3rd party (Google) to implement something that might not be anywhere near the top of the paid (or even volunteer) developers' priority list.
The total stipend to a successful GSoC student is $5500 (and that's not counting the money Google gives to the organisation (e.g., travel stipends)), so that might work as a way to calculate costs - GSoC is ~3 months of working, so that's somewhere between $1500 and $2000 USD/month. As a baseline. If you're hiring an established professional programmer, costs might be higher.
But yes, it is absolutely possible to pay for new features, in the manners @chirlu listed.