Yes, if you are able to buy in bulk then that tends to be very cost effective. I find with a small number of rodents it's easy to buy cost effective food in small amounts, and if you have a large number and can buy by the sack then that's good too. But if you have some sort of a middle number it can be tricky both to source suitable amounts and to get value for money!
I buy barley from the local feed shop, but if you're doing smaller supermarket amounts then it tends to be hidden in the aisle with lentils and broth mixes rather than with other grains.
I buy rodent food ingredients in bulk sacks (10-20Kg) for the most part. I have large storage containers for each of the ingredients, and then mix up specific mouse food batches every couple of months in a smaller tub that is easier to feed groups from. Each batch varies a bit but currently it's:
5Kg pig grower pellets
4Kg premium depurative pigeon food
2Kg HBB (barley-based muesli rabbit food that most of the rat fancy use as a base)
800g white rice
1.4Kg dog food.
The specific analysis of the above mix overall is 15.56% protein and 5.39% fat. I have a big spreadsheet so I can input the ingredients and nutritional values so I know exactly what I'm feeding at any one time (and what past batches were) to monitor condition and react accordingly.
In any case, my general batch base is a mix of pig pellets, low-legume pigeon food, and usually some chicken corn (I sometimes buy one which is wheat and maize, sometimes buy one which is barley and maize). With dog food (usually working kibble as that's VAT free in the UK) and other bits and bobs (e.g. cornflakes, porridge, rice, nuts, dried fruit, any other small bags of commercial rodent food I'm given) added.
This is a photo of the batch of food I made up before the current one
Hope the slower reply is still useful; I don't check this forum regularly.