Most shops charge 50-60 bucks an hour..remember they have to warranty the job. Parts cost for the gaskets, fluids, oils, lubes, BOLTS , and misc can be 200 bucks. Labor will depend if the car is stock/modified/has rusted broken bolts, stripped threads, etc..
And we havent talked bout resurfacing the heads.. valve job.. pull the oil pan to get graphite out..retapping the bolt threads..etc..
We havent even discussed retorquing..
Its a good days work to do the job..and do it rite.. 900 sounds fair if you think about all the steps needed..doing both heads.