That's not automatically the right answer - there are many variables and having a service contract does have some things going for it. But, the people selling these are in it for a profit, so on average they will expect to charge more than it costs them to provide the service.
Possibly the biggest factor is the owners attitude to risk. When (and I do mean when, not if) the boiler breaks down, then under service contract you don't have any unexpected and potentially large bills. Many service contracts also include an annual service, and if you consider this a good thing to have done, then you can deduct that from the cost of the plan when deciding if it's right for you.
Also factor in how badly a failed boiler would affect you. If on a shoestring, and prepared to wait, then you may get cheaper rates for ad-hoc repairs. However, if it would be a serious issue (such as having vulnerable people and/or children in the house, and a combi for hot water) then being able to call someone out on contract may be attractive.
So if you aren't risk averse, tend not to bother with annual services, and have enough cash reserve to deal with unplanned bills, a service plan probably isn't for you.
If you would struggle with an unplanned bill and/or are risk averse, then a service plan may be right for you.
Look at a selection of vendors, and read the small print to be sure what is, and more importantly what isn't, included. In particular, see what local outfits can offer - hint: it may help to ask local letting agents who they use.
I have a couple of rental properties and so by law I have to have a service contract for the boilers. This costs me £15/mo (£180/year) but does include the annual service and safety check - so in effect the "extra" cost of the service contract is only about £10/mo. One of the properties has an ageing boiler - some years I've been quids in on the plan, other years it's just been a cost.