Personally I choose aspen for the rats as I buy it anyways for the snakes and I've found it just as nice to use with the rats too. The breeding females especially seem to like it as it's very light and small shred so they can push it into lovely mountains to make cosy nests out of for their young.
Just another thought....as rats are omnivores just like bears and humans...you might want to suppliment that dog kibble with other things that are just in your house anyways. I toss some left over cereal, dry pasta, some trail mix into a plastic container and just give it a shake then add a bit of lab block and some dog kibble and feed that to mine. They also get leftover salad, bones from dinner, table scraps of all sorts.
I'll have to actually do a cost breakdown on my feeding costs per rat produced but I think it must be pretty minimal as most of what they are getting is just leftovers other than the lab block and kibble. Nice of them to turn what might end up in my garbage can into lovely baby rats for my snakes and save me a ton of money monthly LOL.
~~Jo~~