Since you have "6-12" PIR sensors do you need a separate conductor for each?
What is the current draw from each PIR sensor?
How far is the protected perimeter from the indoor location of the Arduino?
Do you want to run the cable through the air (Aerial Outdoor Cable) or underground (Direct Burial Outdoor Cable)? You can get either kind of Cat5 UTP (four pair) cable in 1000' reels for roughly $100.