I had a couple of local alarm companies round to quote and the price was 700-900 for 6 PIR, a contact sensor, alarm box, and a control box with a dialler. Both were wireless and the control boxes looked dated.
In the end I dis some research and purchased a Alert Gen G7
from Quick Safe
It was £400 for the box, siren, 2 contacts and 3 PIRs. I also got a 3 extra PIRs, a extra siren to fit inside the garage to scare any scum away, and a smoke alarm. It has a GSM module and is easy to setup remotely using SMS or the smartphone app. It you use the proximity tags you can even set it up to tell you when the cleaner arrives, kids come home etc. The siren is solar powered too so no need to get up on the ladder every year to change the batteries or have complex wiring to it.
OK, it's not the cheapest but, it looks good (so many hade awful control boxes and remotes) and can be set up in a few hours. I can alarm the garage PIR without alarming the others.
Wired alarm probably better if you have the time to take up floors to wire it up.