My house was built in 1956 and it was wired with an older type of Romex that did include a ground. I believe all of the boxes were grounded but the outlets were the old 2 prong ones.

I did not change the outlets, I believe the realtor hired someone to do so before I bought it. But so far everyone I checked (which is most) has a pigtail to the ground wire. Unfortunately they used the stab connectors for the hot and neutral instead of the screw connectors and I had to re-do most of them not long after moving in.

So to answer your question, if the house was wired with grounded Romex, grounding the outlets will likely not add to your cost. If new ground wires need to be run the cost could add up quickly depending on your home.