Steam locomotive operating costs

The recent thread about the Eureka 4-4-0 got me thinking about the daily operating costs of steam locomotives. I think there have been items posted in the past, but in terms of current figures, what is the daily cost to operate a vintage steam locomotive and how is that cost broken down?

I remember seeing a couple paragraphs in a magazine years ago where the Valley Railroad said something to the point that a 70-ton locomotive costs X-dollars to run per day. If that can be substantiated, could you use the logic and take that formula to generate an operating cost for a steam locomotive of any size by simply increasing or decreasing the tonnage to suit the particular locomotive?