Guinea pig food problems.?

My guinea pigs are on Muesli (food mix) and I would love to put them on Pellets/Nuggets however I can't afford it as I have 7 and they go through a big 15kg bag of food ever 2 and a half weeks for �10 a bag, and nuggets would be �17 to �20 a bag of that size:/ I can't mix the muesli and nuggets together as I see no point so please don't suggest that. i need some advise please!<3