Spaying is anywhere between $80-$200, while neutering is usually under or around $100.
Small animals are much harder to work on for the vet, and take a lot more time and consideration - thus why the prices are higher than dog/cat sterilizing. Make sure to find an EXTREMELY rat-savvy vet - just any ol' dog or cat vet won't do. Rats are exotics and need special care.
I have 13 rats. 10 of them are spayed/neutered, 2 girls are a little too small for the surgery just yet, and my old boy (Mulligan) is way too old. Spaying and neutering is a great thing to do for your animals, especially the females. It greatly reduces mammary tumors and ceases the possibility of pyometra.