Auto alarms

Greetings, I am self-electrical, I recently acquired a voyager, without alarm, I found several types on the Internet and acquired it at $ 320.00 pesos. That same alarm placed in specialized places, it comes out at $ 1,200.00 pesos, they charge from $ 700.00 to $ 800.00 pesos just by placing it, they sell you the idea that if you do not install well you burn the ECM computer.