The rates on Sandown website are:
Microlight £7.00
Gyrocopter £7.00
Single Engine (up to 1200kgs MTOW) £12.00
Single Engine (up to 2000kgs MTOW) £18.00
Twin Engine £25.00
Helicopter £20.00
Motor Glider £20.00
Touch & Go 50% of applicable landing fee
Touch & Go Hourly (or part thereof) £25.00
Have you asked them why the motorglider rate is such?
(I can see that some C42 pilots might, for instance, wonder why a light aircraft one is £12 whereas a microlight one is £7, and so on, but price lists are ever thus.)
Our overnight hangarage rate - for instance - probably doesn't cover our costs, since we have to insure for the maximum value in the hangars on any one night for the whole year. In other words, if we have #1 million of aircraft based here and someone wants to park a Spitfire indoors for just one night, we have to pay an annual premium based on £2 million!
But we have chosen to absorb some of this. (Although you might still be asked to park outside if visiting!)
There may be some hidden costs for motorgliders that we don't know, or they may just be a pain due to their wingspan!
And do you really feel £8 is worth rejecting the pleasure of flying to a nice airfield?
Try parking a car at Glasgow or Edinburgh airports!