The regulators are rediculously inexpensive. You can get them for like 35 cents each online. And you won't need the large 4700uF capacitor since you've already got the 7.5V DC. You could use a much much smaller and cheaper cap, like a 22nF.
Do you have a voltmeter? If so, and you haven't gotten the regulator and such yet, why don't you just go ahead and put on a 80mW load at the end of your 20m wire you are planning to use, and see if the voltage is acceptable. You'll be sending 7.5v DC down the line, so use around a 680-700 ohm resistor (that's not a link why does that always happen :?). If there's very little drop, and it's still close to 7.5v, then we can assume that you will get a similar performance when you regulate it down to 5 volts. When you do use this single central regulator method, and branch out the voltages to all those long wires, you are going to want to put a filtering capacitor at the end of those long wires so the circuits there get clean voltage.