If they billed 7.5 million at a 3% merchant rate this mistake will cost them $225,000 in merchant fees. Of course you would need to factor in somewhere around 2.5million in papal that's unaffected and about another 20% for cards that declined.
I'm assuming they voided out all the charges before they settled. If they didn't they will be paying an additional 3% to issue refunds.
My guess is that they will have somewhere around $10,000 in reimbursing to do for overdraft fees.
The amount of chargebacks they have this month will probably 10x resulting in $10,000's in additional chargebacks and fees. The crazy thing is that you can actually do a chargeback even if you already got your money back from a refund. It happens to us all the time....
The customer loss from it will probably be significantly less then one day of signups for them. The real damage is going to be with their reputation and trying to catch up on work load.
Dreamhost is having one of the worst days they are ever going to have right now. If you were one of the people affected try to go easy on them while they get everything under control. I'm sure in a few days times they will have everything corrected and will reimburse you for any losses that have occurred.