I think Firepower is a really great game, especially in its day. But its day was over 40 years ago.
When you start pushing the price to $6k, there are just too many other games that provide a ton more to do at a similar pricepoint.
One can try to justify the price by how much was spent restoring, but that is not how a buyer determines the value.
Firepower is a cool game, but it's still just a step above an EM in depth.