ADT service is usually around $36 per month (in California) when you get new service with contract. They usually give you the equipments for free if you sign up for 2 or 3 years. Installation fee is $99. They are not tell you over the phone because they want to upsell you during home consultation.. extra remote, camera, etc... and they want you to get into a contract with them. Since you already have the equipments, it is harder for them to talk you into a contract so they have to come face to face and do their sales thingy.
You cannot use the equipment without service, but just having the ADT sign outside probably did 90% of the job. There are cheaper service out there that can use ADT equipment around $20 a month I think. You have to search for them.