There is always someone out there ready to rip off the unwary. Think about it, a spark plug is just a bit of metal with some ceramic insulation, i.e., raw material cost is pence add on cost of equipment for making them by the millions plus packaging and I would work hard to find them for a fiver each and this is why I never buy anything from "dealers" unless I have no alternative. Here's a good example:- As can be seen from above table "dealers" charge at least £489 for a cam belt change which is ridiculous for something one can buy for around £100. My super efficient and lovely mobile mechanic (no VAT) recently quoted me £250 for the job.