I learned over the course of a few weeks, that it is much more economical to use less wood-pellet bedding, as opposed to overdoing it, with rats. I use a thin layer; only 1-2 pellets deep. But when I was using it deeper (like 1-2 inches) I was getting a bunch of sawdust on top with still-good, yet unsalvagable, whole pellets underneath. What a waste!
In using less, I save pellets bigtime. Also there is less likelihood of the water-bottle nipple getting buried in pellets and creating a sponge effect that sucks all of the water into the pellets, leaving your rat thirsty and your bedding ruined (this happened in one of my females' tanks; luckily it was only 2 hours or so after I changed the water so they was not without water long.)
I do add in some cloth (socks; the cheap, irregular ones that you can get cheap at the dollar store under the Oops! brand name.) They give the rats, especially moms, something to sleep on, snuggle in, and nest.