It's an open country park with a golf course in the middle ... the farm aspect is only part of their setup. I honestly don't see that charging access would be feasible - it would be more likely they'd shut down the country park aspect as well, given the livestock area directly beside the visitor centre and parking area is a major pull for young families from the area who just wouldn't visit if they had to pay to get there.
Yet there is so much missed opportunity to generate revenue. The visitor shop is a joke - the cafe no better - the garden nursery shop I have yet to see open - and they could easily run a series of courses or information talks with a nominal charge, yet I've seen nothing advertised, so if they do, they don't want you to know.
Compare with Mugdock Country Park in East Dunbartonshire ... a great visitor shop that offers hand crafted products by local people as well as the expected tat - a cafe which serves really good food, much made fresh by them each day - countless events through the year, both free and for a small fee - hire of a couple of their rooms including a very well equipped small theatre style room - an outstanding privately owned garden centre ... the options are there, but they're just not being done at Palacerigg.