I'm getting a quote from a mechanic of around 900 to replace the brakes on my O2 Escalade EXT I'm just curious if that's a fair price or not

NO.get more estimates

Define replace brakes in more detail... There's a lot more possible scenarios to a brake job than just installing new brake pads!! .. What 'exactly' (parts & labor) was the estimate for?

go to estimator on this site for your area.it all depends on what is being replaced

did you check with repair pal? for you area

He is robbing you go to another mechanic. Try to find a mobil mech. Also maybe get the front brakes changed one day then come back and return another day to get the back brakes changed. You will pay way less by doing it separate than all at one time. The mechanic is over charging you because its 4 wheels done at one time.