Wow - quite the layout!
The value of the layout itself is probably quite small, since it will probably have to be destroyed to remove it.
HOWEVER -
The rolling stock (cars) and motive power is another matter, along with the structures.
Manufacturer and type will have a lot to do with the value. For example, if the steam locomotives are brass, there is significant value there. At the opposite end of the spectrum, if they’re old Bachmann and Tyco, they have only nominal value. Rolling stock is a bit easier - at train shows you could expect to get around $5 to maybe $10 each for the cars; a bit more for some, and a bit less for others.
Structures are less dependent on brand, but might be harder to value anyway. From what I can see they’re nicely constructed, so the larger, more complex ones might be worth a decent amount of money if they can be removed from the layout without damage.
Switches (track turnouts) may be worth a few dollars each, if they can be removed from the layout without damage. Depends somewhat on the maker.
(you wouldn’t happen to be in the vicinity of Wyoming, would you? I wouldn’t mind seeing this layout before it’s disassembled).
Check out the model railroading items for sale on ebay to get an idea of values. A lot of the stuff you can sell there for a fair price (and sometimes a bit more than a fair price, if you’re lucky!).
Another option is to visit a few model train shows in your area. Scope out the prices they’re getting for items similar to what’s on the layout, then sell the stuff at the following show, or another one nearby.
From the photos, and just off the top of my head, I’d say you’ve got somewhat more than a couple grand worth of stuff there if you sell it piecemeal.