As part of a BBC investigation into bank fees, the experts gave the figure after banks refused to detail their costs, despite charging customers up to £39 for going overdrawn.
It is already bad enough to enter into an overdraft, but to have a penalty fee added to this can make debt management even harder.
“The banks, I think, are putting these charges on to make a profit, no one would agree with the charges,” said consumer Jemma Miozga, speaking to the BBC’s Money Programme.
“They are there for the banks to make a profit and it’s not fair. I just thought it was what banks did. So, I was astonished when [experts] said [the charges] were illegal.”
She has since claimed back just under £5,000 in charges from her bank, Halifax. Consumers can often reclaim such fees through the small claims courts.
Kieron Beal, a barrister from Matrix chambers, added that he found it “odd” that banks do not defend such actions, meaning that there is some good news for consumers seeking to recover charges incurred through a lapse in their debt management.