I think a basic starter set like have (which had a lot added to it when I was younger), runs in the $400 range. The set includes an engine and two cars, as well as enough pieces of curved track to make a circle about 4-5' in diameter.
The track is what is the killer, IIRC. It doesn't take long to get a bunch of money wrapped up in track. I don't know current prices on track, and I don't remember what it ran 15 years ago either, but I remember $80 in birthday/Christmas money didn't get me home with much back then.
I'm sure it wouldn't take too long to get a couple of grand wrapped up in a system.
For the right client, I'd jump on the opportunity to install one. But I would want to consult with someone along the way too....
Think about it, how many higher-end clients have poolhouses with their pools? Have someone "tending bar" inside the poolhouse during a party, and deliver the marguritas via train set. I don't think the novelty would ever wear off.....
Enough of the rambling!