I don't think there is a standard price, as it depends on the cost to run the train, how many people you will be sharing the experience with and how unique the experience will be.
For big steam, PM 1225, MLW 261, etc., it will cost in excess of $10,000 per day just for the locomotive, so you can do the math. Trips like this often run in the $300 per person per day range, but they don't happen every day.
The record holder has to be Dave Goodheart's never to be repeated 3751 trip to the Grand Canyon which if I remember correctly was around $600 for one day.
Any all day charter for $125 has to be considered a deal these days!
Michael Allen