It is not legally binding. It is morally binding. People will have different opinions about what constitutes a good enough reason to back out of a commitment.
Personally, I think you should make a very strong effort to find out the estimated cost before you ED, and only ED if you are definitely comfortable with the cost.
You should not ED if you are counting on uncertain Merit aid to make it affordable.
If the actual cost comes back very different than the estimated cost, you should be able to go to the school’s financial aid office, and figure out if there is a solution.
But, in the end, if it is going to cause your family financial hardship, I don’t think you should go through with enrolling.