I took out a 4-year Service Plan at the beginning of February this year. It's £27 per month. The first service was due at the end of last month so I had to pay 3 months up front (£81) to comply with the T&Cs that say min 3 months payment before a service is due.
R2D2 and I have the same engine (6 pot 3.5 litre petrol - ie the best one!) so our service costs would be the same. The fixed price servicing that is shown in a booklet I was given by my dealer says:
Minor Service £269.00. Major Service £369 (no matter which younger C-Class - but interestingly they're now charging an extra £20 on each service for their "Drive or Collect Package" that includes use of a courtesy car or collection and delivery, I don't know if this is covered by the Service Plan but they ferry me back and forth so I don't need it).
It's the "Additional Operations" that really add to this. So for the 350 engine the extras are:
2nd Service: Brake fluid £59
3rd Service: ATF & filter: £251
4th Service: Brake fluid £59, Spark Plugs £175
So over 4 years that's a total of £1820. My Service Plan works out at £1296, so a big saving. An even bigger saving when you assume that service charges will be going up over the next four years! (The savings are slightly less for 4-cyl cars, but still worth it.) I haven't looked into what the advantages are for high mileage users because I don't fit into that category.