I would expect W or Y to be more expensive than Z rated tyres, but you never know. Y (186mph) is higher rated than W (168mph), and Z is peculiar being rated at 150mph+.
Mercedes "MO" fitment are usually W for summer tyres and H for winter tyres. I opt for Y rated summer, and V rated winter tyres though.
Don't be tempted by V rated summer tyres though unless you can set the speed limiter, as they're rated lower (149mph) than your vehicles top speed (at least 155mph), and that could get you in hot water with insurance in the event of a claim.