As milliways has answered you will get a steep voltage drop on such a distance using 5volts.
This is common problem in all long distance power transmission.
The best solution is to use a 24 Volt power supply and use a 5 volt regulator on the Pi side. There are cheap modules available or you can easily make your own regulator using a capacitor and a voltage regulator 78xx. (Sparkufun) (Wikipedia)
But it would be more efficient to use a switching regulator V78-1000 if you want to save power.
The reason using 24 volts is better is:
- The obvious voltage drop problem on a long wire.. this wont matter if you regulate down to 5v
- Higher volts does not need higher amps (its actually opposite). So you can use a 0.5amp power supply but when regulating you will inface get more amps to use on the 5v side! This will help use a thinner copper wire (like network cable) which will reduce the voltage drop (less resistance)
FYI - Some POE standards uses up to 50 volts to power remote devices.
Just a side note to consider but not critical - As commented by SF. It is better to send AC current- but 220volt might be dangerous so a compromise is sending 24volt AC and rectifying (also easy and cheap using 4 diodes) and regulating it at the Pi. That is because of various inductance problems and stuff I am not really all that familiar with.