5V DC 500 mA adapter run 100 feet to web cam
Not knowing what gauge wire you will use, I guessed 18AWG. At 100 ft (200 round trip), a 5V source voltage at 500mA will result in a ~13% Voltage drop at the far end. That is outside of the normal USB spec of ±5%, which is 4.75V to 5.25V to be in spec. It might work, it might not. You can either increase the source voltage, or increase the cable size (16AWG results in a 8% drop instead).

Additionally, as the current pulled increases, the voltage will drop more. V = IR after all, with R being the resistance of the cable (normally stated per foot). If the extender + Web cam actually pulls more than 500mA, this will change. If they pull less, the Voltage at the far end will be higher as well.