I had to get mine off Gertie onto something else because it was too costly. I remember a 15kg sack being about £15 really not that long ago. Seemed to jump to £18 then up to £25 plus with no steps in between. Mine eat Tesco food now and love it.
I don't know if this is any help I have no idea how much it normally costs
http://www.vetuk.co.uk/guinea-pig-s..._667/supreme-gerty-guinea-pig-original-p-2107 £25 for 15kg
I'm using this atm which works out really reasonable
http://www.chemistdirect.co.uk/wagg-optimum-guinea-pig_1_49164.html
I've taken mine off Gertie now too, partly for cost, partly as I never realised how many colourings were in it. It also says on their website only the red flowers have the added vitamin C...well half of mine left them, so that wasn't really ideal!
I managed to get a 15kg bag for £15 in P@H as they labelled it wrong, haha! But not seen anything less than £25 since.