Upload & Sell: On
For a 50th wedding anniversary, I'd cough up the $1800 and try to take the first-class, customers-above-all road in customer service.
First and most importantly, I think it's the right thing to do.
Second, you may find that, at the end of the day and if things go well, they may be willing to chip in to cover some of those costs. If things don't go well, your thorough and dedicated efforts to recover the data may prevent a raging client, or someone who tells everyone they know just how badly you screwed them, or a negligence lawsuit. Any of those would cost you more than $1800 in money and lost business.
Third, if things do go well, and even if you do eat the $1800, you could potentially recover multiples of that amount from recommendations they make to friends. This goes with the prior point: whether it ends well or badly, this is exactly the kind of story that people will tell to all their friends... so try to make it end well.
As a totally separate note from this, and eschewing the obvious "duh" notes about your need to improve backup and verification procedures, I'd also recommend that you look into the kind of insurance coverage that could cover the costs of either the $1800 recovery fee, or some kind of compensation to your clients. And be ready for next time, because someday there will be a next time.