A BIT expensive?!?! what car was it, a Lamborghini?
A set of decent quality Denso plugs for my Punto cost me under a tenner delivered to my door; an air filter is under a tenner; all can be fitted in under 5 minutes!
Did they come attached to a new engine by any chance !!!!. Did you get an invoice for the work and if so what was the breakdown. If they put in too many hours then you have a good case to get some money back.