They don't all cost that much, but $150,000 seems to be a reasonable starting point. They have a storied history and most of them saw a lot of use.That link has detailed pictures of the gun from various angles and you can see how much wear it shows.
A nice reproduction can be had for about $400 and if shooting it was part of what you want it for, that would be the way to go. The originals are 160 years old and weren't made to an impressive level of metallurgy when they were new.
Of course, a $400 reproduction isn't a grail gun.