If anyone offers different levels of servicing - they're a cowboy outfit - tell them where to get off. Unless it includes adding/checking inhibitors etc etc.
A service is a service and is required to ensure the safety and prolong the life of gas appliances. It should include the cleaning and inspection of parts and inspection of flue, flow tests and spillage tests - replacement of gaskets, flue gas analysis (where necessary) simple as that.
You need to supply more info like what area you live (obviously cost changes around the country). And you should ask what a basic service is? I have never of different levels of service but maybe they are trying to sell you some useless extras.
Also the cost can/will go up depending on how many gas appliances they service.