Put it in the ad. It's not common, but it's not super-rare, either. And the main reason you don't do it is that you don't want to eliminate your chance at hiring someone at a below-market rate (or of alerting your below-market-rate current employees that they're being screwed and should be getting big raises). If you're committed to paying the rates you'd need to pay to get good talent, then put that upfront.