Wireless Security System

Hey, this is the first time I post in the Arduino forums and i'd consider myself a novice when it comes to programming/electronics.

I am currently trying to develop a security system for a relative's restaurant. He wants it to send him a text message when someone enters the restaurant either from the front door or back door, and requires the motion sensors to be wireless so there aren't wires spanning the entire restaurant. I want this project to be as cost-effecient as possible beause he does not want to spend a couple hundred on a pre-build security system.

From the list of requirements, I know I will need a GSM shield, at least one Arduino, and at least two motion detectors, but how will I make the whole system wireless AND cost effecient?

I'm not a wireless expert, so I can't help you with that. It sounds like you need two kinds of wireless communication... You need local communication between the sensors and your "control center", and you need communication over the cell phone network.

I think this is all feasable, but even if you build it yourself it's still going to cost you a couple hundred dollars. Most of the time, my little Arduino projects end-up costing about $100 USD buy the time I add some additional electronics and connectors and put it in a box with a power supply.

and i'd consider myself a novice when it comes to programming/electronics.

I'd recommend you buy most of the system. There are some inexpensive wireless home-security systems. You can probably build the GSM part of the system.

Now... One of the first projects I made with microcontroller was an alarm system for a vehicle (with none of the wireless complications). But, I had quite a lot of electronics experience and some programming experience... And, it was for myself...

An alarm system for a business is "serious business" and it's not really something for an amateur. False alarms will be annoying and if it fails to detect a burglary it will be very-very annoying and expensive.