Parts purchased so far for my '93 500SL:
Window motor and gear assy: £120
Distributor cap: £80 - needed two
Distributor arm: £20 - needed two of course
Oil semi-synthetic: 8.5 litres - round about £4/l - £34
Oil Filter - £20
Pollen Filter - £25
Infra-red key - £55
Wiper Blade (not just rubber) - £20
All purchased directly from MB.
I've been pleasantly surprised as parts (so far) haven't really been much more expensive than for my 190E. I'm sure that you'll find that service parts are really no more than other cars and probably cheaper than many Japanese models ;-)
Lea