Start with the list of covered systems on any service contract. That list is where the price comes from. Each system named adds to the cost of the plan, and each system removed takes some of it away. The total moves with the list, so the list is where your review should begin.
Read what is on the list
Read the covered-systems section before anything else. Note each system named, and note whether it is a whole-house system, a single appliance, or a component inside a larger system. Check whether the contract lists systems by name or by category. A category line can include more equipment than you expect, or less. Ask which reading applies.
What each added system does to the total
Adding a system usually raises the price of the plan for its term. The increase depends on how the provider prices that system, not on how much you want it. Ask how each added line is priced. Find out whether it is a flat line item or a change to the base plan. Then compare the cost of the addition against the repair cost you hope to avoid for that system.
What dropping a system does
Removing a system lowers the price, but it also removes that system from the repair coverage. If the system is one that is likely to need a visit in your home, dropping it moves that cost back onto you. Look at the age of the unit against its planning window. The central air conditioner planning guide shows typical service life and when to plan, so you can use that information when you decide whether a system stays on the list.
Ask about the rules that apply to the whole plan
Changes to the list often come with rules that apply to the entire contract. Ask whether the list can change in the middle of a term. Ask whether an added system starts right away or after a waiting period. Ask whether a removed system can return at the next renewal, and whether the price is fixed for the term or can be reset when the list changes. Get those answers in writing and keep them with the contract.
Compare the list against the house
Make a short table of your own. List each system in the house, its install year, and whether the contract covers it. Mark which systems are well along in their planning window and which are still early. The heat pump guide and the gas furnace guide each show the typical service life used for that comparison. The appliances ranked by typical service life page gives a quick view across the household appliances. Systems further along in their planning window deserve closer attention in the decision.
A longer list is not always a higher total per system
A longer list does not automatically mean a higher cost for each system, and a shorter list is not automatically the lower cost choice. Compare two contracts on the same list of systems before you compare totals. Otherwise the difference may come from scope rather than from price. The published figures for each system sit in its own table on this site, so you can check the planning dates before you compare anything.
Before you sign
Read the list, the terms for adding and dropping systems, the waiting periods, and the rules for changing the list in the middle of a term. Write down the total for the term using the same list you plan to compare. Keep the written terms with the contract. Those steps set what you will pay.