I have fed my rats and mice most every commercial diet going over the years, and the diet I feed them is based on the ingredients of my favourite mixes, and a particular home-made diet popular in the UK.
The main ingredients are:
5 scoops (20%) - Flaked peas
4 scoops (16%) - Flaked wheat
4 scoops (16%) - Flaked maize
4 scoops (16%) - Bruised oats
4 scoops (16%) - Flaked barley,
3 scoops (12%) - Dogfood (whatever brand I am currently favouring)
1 scoop (4%) - Parrot food (mixed seeds and nuts)
I also add some dried whole peas, pasta, bits and bobs of other straight cereals, and other things I pick up at the wholesalers at a random basis, to add a bit of interest. For mice and multimammates, I also give sprigs of millet on a regular basis (once a week or so).
Fresh fruit and vegetables - I don't tend to feed fruit, but vegetable wise I give mainly three different things - curly kale, broccoli, and carrots. Curly kale provides an incredible amount of calcium, a trace element which is lacking in practically every captive rodent, and I consider it to be essential. It's not always available, but when it's not it can be substituted with spring greens or dandelion leaves. When it is available I buy it all and freeze it, it can be fed straight from the freezer.
I think choosing a food, especially for feeder rodents, is hugely important. I will not feed my rodents anything with artificial preservatives or colouring in - most cheap dogfood contains BHT or Ethoxyquin (this does not have to be declared on the label). My concern is not only the initial health risks to the rodent, but the chance that these chemicals will build up in the snakes fed on them - maybe no short term effects, but I don't feel it's an area where you can be too careful!
Even choosing to use a premium dogfood, this diet actually works out a lot cheaper for me than buying sacks of "rodent lab blocks".