As Gibby said, it will come down to how much you want to spend.
Personally, I will always spend what I can afford, plus a bit more - if your car represents a theft risk, the cost and inconvenience of a break/theft outweighs your initial outlay.
Looking at that link I would personally be very sceptical. On the basis that most Cat 1 alarms are ?250+ an alarm for a bullseye is likely to be a bit pony.