I would say that €8 is extremely cheap and €30 is very expensive .The shop I'd normally use charges €8 to change over a tyre
My local suburban shop charged me whopping €30 which I didn't expect
if you are not willing to give the shop the business of buying the tyre from them, they are entitledd to charge a bit of a premium for fitting only. They have expenses to cover. I am always amazed how cheap bike repairs and servicing is.