If Hal was mining ever since the start of the network, this will put him to the hundred thousands of bitcoin on the safe, if not million/s IMO, so I don't think $12000 is the right valuation for what he has in store for his heir. What the $12000 stands for, perhaps, is the donation they received when Hal was still alive.
In the linked post he talks about mining for a few days and then stopping, not liking the effect it had on his computer. He then writes about forgetting about Bitcoin until over a year later.
I'm surprised at that as I remember him saying he'd been waiting for many, many years for something like it to arrive.
At the time it would've been just another project popping up, albeit an extremely interesting one.