How Much For A Burglar/Intruder Alarm?

I am thinking of getting a burglar/intruder alarm for my mionette but dont know how much it will cost and who is the best person to do it. I aint got alot of money so if I could pay weekly to have one fitted that would be even better. I have a cat so the alarm has to be pet friendly. I live in Bradley in Bilston West-Midlands so if anyone knows of any good companys near there or any that are good and have reasonable prices please let me know. Any websites or prices that you may have paid too please.