There were people who made a decent amount of money in the first couple of days of the RMAH. That will certainly go down as competition goes up.
Blizzard takes a pretty big cut. I read a funny post here Diablohead about selling something for the RMAH minimum ($1.25) and coming out with a whopping .25 cents. Blizzard and Paypal get 1 dollar. You get 25 cents. EXCELLENT!!!!
Yet another reason I will probably no be doing anything on the RMAH. I won't say I will never use it, but as of right now, it is not looking good. I will never buy any gear off of it.
This is going to be a great way for Blizzard to make a killing.