All home educated group visits are free of charge during term time for their guided farm tours and general visit.
They charge for facilitated workshops ie butter making, field to fork etc. http://www.nms.ac.uk/about-us/schools-programme/school-visits/schools-visits-to-national-museum-of-rural-life/
The workshops cost £70 for a full class (max 33 pupils) but they also have a half class charge and a single pupils charge so the cost would be calculated according to number of children attending.
They also have a lunch room that can be booked for packed lunches.