Entry to the event, cafe & toilets on the day is FREE.
And as if that wasn't enough drivers & passengers arriving in a kei car will receive vouchers giving them Museum entry on the day at £5 per person. Please explain you are with Kei Kars in the Park to qualify.
I believe speaking with John @ the HMC that all cars that are kei cars, over 20years old OR future classics will get the discounted price for the museum. So if you tell the marshals that you are with KKITP then you should qualify