At the end of the day - if you dont like it then dont fly them - no one is forcing anyone to fly Ryanair. Personally I can see where this cost is coming from, its not cheap to have a plane serviced on the ground! Also I think Ryanair (never flown them so Im not 100% sure) still uses waste liquid rather than the newer vacuum system that for example the A380 uses. The waste liquid is also quite expensive for the airline to buy and supply.
Regards, James