been a while since we had one of these, please cheer me up - just worked out feed costs for winter and in some ways would be better off on full livery with all feed included, but has been a long time since I worked out £ per pound costs 
but unlimited great quality hay £5 a week makes up for a lot
so DIY grass weekly for me:
£25 per horse for field, hay, water, electricity
£23 feed pony
£35 feed horse (weeps)
large bale shavings £7.50, if put shelter up, no immediate risk of that
all seems very £££y for DIY, even with use of one normal sized outdoor school and a cross country course plus great hacking