Like $20-25. Sometimes more or less depending on your vet.
It's similar to deworming puppies.
I use horse Dewormer paste or Ivermectin from the feed store because it's easy to get and is cheap. It also works really fast.
It comes in a big tube and I just use a needless syringe to give orally to the rats. The dosage is about .01-.02 cc per rat (rice grain size).
And you shouldn't treat pregnant/nursing mother and pups under 2 weeks old.
(Ivermectin can also be injected subq if you wanted)