The place that used to groom my kitties before they closed only charged me $15 per cat if I brought more than 1 at a time.

The place that did a crappy job was one of the cheaper in the area and they charged me $35 for each short haired cat, and $40 for each long haired cat, no discount on multiples. That was about 4-5 years ago when I quit using a groomer though, so I'd imagine the prices had went up some since then. Don't know what they charged for a dog though.