It's worth less than a similar condition .41 caliber of the same model. The .41 is a Prescribed Antique, at least if it is pre 1898, while the .38 cal ones are registered restricted firearms, courtesy of the caliber being on the list of 'bad' calibers.
The price ranges you see are pretty close to what I have seen at gun shows. Guys can ask the moon, but what it will actually sell for...
The innards on the Thunderer/Lightning pistols are fragile, lots of small pieces, and they don't respond too well to a heavy hand. PITA to assemble correctly too, as the parts must all mesh properly to work.
Price on that one? I have seen a few similar condition, asking from $450 to $650 range. I'd think that it'd have to be a bunch rougher than that one to merit a lower price. But that's just my opinion.
Take a look at Colparts.com and see what the ask is for used parts. It'll give you an idea what i may be worth as a wrecker/donor piece.
Cheers
Trev