I used to feed my previous pony hifi lite and although we bought in bulk i think i did come to around the £6-£8 maker per bag. I would give her around £7 for it, even if that isn't the full price you have paid roughly what you should do.
Threeponies....i personly think that charging more than £7.50 would be paying too much but that is just my opinion. Unfortunatly the feed shop near me is only a 3 min drive but is soooooo expensive so we have to drive quite a while to get to out regular one, although we did pay more petrol wise it still didn't cost as much...or at least I don't think so.