Hi,
It depends on the type, make of the alarm and car really. Best thing is to ring around the varoius alarm places in your area and ask for quotes. Some places will supply and fit others are happy to fit any alarm you bring along.
Last one I had fitted cost �200, for a Cat 1 alarm / immobiliser.
Also did a DIY one and frankly I wouldn't bother as it took hours in a cramped position under the dashboard and bonnet, skinned knuckles, frustration at not having the right connectors etc...BTW have you seen the weather out there this week? It's your choice really.