How to reduce cost of a micro-controller based security systems?

what do u mean by reduction in cost do u want manufacture circuits on industrial scale? or do u want reduce the cost in terms of optimizing the design for low cost? 

One efficient way is to let the controller go to sleep mode when it is not receiving or transmitting any information. In this way you can optimize the battery consumed by the controller.

Try to use the latest sensors which consume less amperes while working and have the ability to go on sleep mode while idle.

I suggest you to shift the components to SMD parts.

If the security systems are sold on bulk quantity then manufacture the kits from China.

Order to make the bodies in bulk.

Hire a team of less educated people to insert the kits in the body.

Hire some Engineers to tune the Kits and for fault tracing.