What is the average cost of new brakes for a 2004 Suburban?

There is no average cost that can be given. Around here (the rust belt) you could easily need to replace the brakes lines from front to back, rear parking brake shoes and backing plates, calipers, pads and rotors, and maybe even some parking brake cables. The trick is you car needs what it needs, and doesn't need what it doesn't and it needs to be checked in order to know. Once inspected estimates can be provided, but remember this, all four wheels and calipers have to come off, and the rotors have to come off of the rear to inspect the parking brake assemblies. That takes time (close to an hour to get it all apart) so it doesn't get done when someone does "free" brake inspections. Find a good shop near you and have them inspect it and show you what it needs.

Around $150.00 per axle with good pads and rotors turned, if no other problems.