Generally, unaffordability is the only reason to back out of an ED acceptance. That being said, if you need need-based financial aid to be able to afford to go, you should run the NPC before applying. If the number given is acceptable to you, print out the results so you have something to use as evidence should the aid not be the same as you thought.
I get the impression from your post you are not asking about need-based aid but are asking about merit scholarships. If that is the case, you may end up being disappointed. The vast majority of schools that offer merit awards do it to attract high caliber applicants that would get in to a “better” school in the hopes of luring that student to attend, so as to increase the stats of enrolled students. They generally do not give a lot of merit money to ED applicants because the ED agreement says you will go if accepted. Additionally, once accepted to an ED school, the student must withdraw all other applications. You end up not having the opportunity to compare scholarship offers from other schools.