With my Jaguar S-Type V6, I only had to change the spark plugs once at the manufacturers recommend interval of 70,000 miles. That was once in the 10 years of ownership.
Though it may be more expensive, I think it best and wise, to follow the manufacturers recommendation relating to replacement mileage intervals etc., and then to use the correct parts specified, especially when it comes to engine parts.
My reasoning for saying this, the manufacturer knows what the stresses and strains are on the individual parts used in the design of their product, and therefore to use what may appear as a cost attractive alternative, it could turn out to be a very costly choice at some point further down the line.. With regard to spark plugs, only ever use a proven high quality grade alternative, to that specified by any engine manufacturer. Remember there is generally a good reason why spark plugs may appear expensive at first site, but it is dangerous to cut costs for such an important item of this nature.