My solution (though it would cost more than $5) is:
Freetronics 4 channel power over Ethernet mid span injector $39.99 NZ. I already have one of these driving an Arduino over Ethernet. It's outputting 9v on the Ethernet cable.
Homemade Ethernet to 2.1mm barrel plug? There are various How-tos on the Net for this. Basically you break out the power wires from the cable and route them to a barrel connector, or you use the cable shown the OP.
I'd use Sparkfun's breadboard power supply to take my 9v and step it down to 5v. It takes the barrel plug as its input. Fortunately I have one of these too, so can save myself a further $16.87 NZ
I'd cannibalize the micro USB plug from an old cable, since buying and wiring one from scratch is not trivial - the connectors are very small. Then I'd connect this cable to the 'out' on the power supply.
My Raspi is a foot away from my mid span injector so voltage drop is not an issue. I could go up to 12v, which my breadboard power supply can deal with, then I think I'd be good for a bit more distance, maybe 10m
I'm thinking of putting this into practice in the next few weeks. Really it's not going to save me much, just one wall wart power adaptor.