hi joules
found this "The Festival directly employs some 1100 people with a PAYE wage bill of around �500,000. 25% of that workforce comes from the local area earning �300,000 - which is over 60% of the total monies spent on wages each year."
on
http://archive.glastonburyfestivals.co.uk/+info/+local/
So if we assume that all 40% of remaining wage bill is for artists you got �200K!
Hope that helps - if it doesnt try emailing from the site they might be helpful as they seem to be very transparent and accountable for everything to do with the festival