Firstly you should decide whether you want to buy & fit it yourself or get professionals to do it. You could go to securwize – home security alarm quick buyers guide for some information. But give yourself some basic knowledge beforehand.
As a rough guide, a monitored smart home alarm system will be the most expensive and a DIY stand alone system will be the cheapest. However, do not expect the same level of control flexibility or protection from a DIY kit.
Saving on the type of alarm could cost you more if your get broken into and robbed!