This project is pretty cheap, especially if you compare it to the cost of replacing your stolen bike! I've outlined the cost of all the components used in my design below. I was able to use some old parts I had lying around and hopefully you will too. If I had bought everything needed brand new this would have cost about $225.
Arduino Uno ($24.95) https://www.sparkfun.com/products/11021
Arduino GSM Sheild ($99.95) http://www.amazon.com/Arduino-GSM-Shield-Integrate...
Adafruit Ultimate GPS Breakout ($39.95) http://www.adafruit.com/product/746
Arduino Prototyping Shield http://www.adafruit.com/products/51
>1.5A PNP Transistor, I used the TIP127 just because it's what our local electronics store had. I'll go over precautions for using different transistors later. ($1.00) http://ca.mouser.com/ProductDetail/STMicroelectron...
2.7K Resistor ($1.00/pack)
7.4V 2S Li-Po Battery Pack ($10) http://www.hobbyking.com/hobbyking/store/__37346__...
Retractable Cable Lock ($12.42) http://www.amazon.com/Master-Lock-4605D-3-Foot-Ret...
GSM SIM Card ($10 + 30cents/sms) This is the rate on Rogers in Canada. Just make sure you are on a GSM carrier.
Under Seat Bag ($14) for Vancouver-ites MEC has a great one that fits everything snugly. I suggest you try fitting everything in the bag before you take the tags off. http://www.mec.ca/product/5024-801/mec-shell-seat-...
5 Minute Fast Set Epoxy ($5)
Zap Straps ($2)
2S Li-Po Battery Charger
Philips Head Screwdriver
Flat head screwdriver (or some other good prying tool)