Spark Plugs –
Each car has a specific Spark Plug designed for it, depending on the manufacturers service schedule. They come in several different sizes so getting the correct size is important.
The main difference between Spark Plug types is their construction material, which can be one of 3 things. Copper, Iridium and Platinum are the 3 materials you will find Spark Plugs made of.
Copper Spark plugs are generally considered to have the best performance, it is the best conductor of them all, but has poor longevity and needs changing approx every 20,000 miles. Copper Spark plugs are the cheapest available, so making them an affordable option.
Iridium Spark plugs and Platinum Spark Plugs perform at a lower level than copper spark plugs, because they are less conductive and they tend to overheat. However, the overall longevity of these two types of metal is better than copper plugs. Iridium an Platinium Spark Plugs will need changing approx every 40,000 miles. Iridium Spark Plugs have good longevity and decent performance, but are more expensive. Platinum Spark plugs have good longevity but the worst performance of them all.
So this is why the prices vary so much, it all depends on the manufacturers recommendations for your vehicle.