they need to make some money out of it after all their doing you a service -and everyone takes a cut of distributions, storage, delivery....sounds about right for cost so your not being ripped off thats just the cost of it, you could just get a different set of tyres like an older generation of the same to get something cheaper -what tyres are they?
there are some terrible sellers on line.
looking at it another way they wouldnt know what or where you got those tyres from and there storage conditions so if there was any fault it would be easy to blame the fitter and they have to sign their name to it...
you could save a few quid over the years of buying the equipment to do it (simple hand operating ones) but that means buying it and storing it in the first place.
if you get to know the guys and be a "easy going regular" they might more often lower the cost or throw in some other nices that others would charge, sometimes it pays off to keep it local woth an independent.