John & Co.
My recollection of the cost for the Promontory point engines was in excess of 1M. I think that was each! Of course, they were being built for the government!
I think the estimates for restoring Cuban and or Brazilian 2 ft. or 2'6" locos is close, but I suspect that building a new one would cost more then you estimated, due to the engineering costs and pattern making costs. A boiler requires the ability to bend and punch metal, but no forms or patterns are necessary. It's the cylinders, drivers and frame fittings that require expensive patterns, unless one went to all weldment construction.
Does anyone know what Crown Metal Works used to charge for their 2 ft. and 3 ft. ga 4-4-0's? A "real" engine would cost more than that, but it could give you a floor number. Also, doing it a piece at a time, over a long time period makes fund raising easier. The British "Tornado" project has been underway for about five years and they are up to a frame, with cylinders and drivers. My dad builds live steam engines and he says it took him about 10 years for each one. With a realistic design, maybe we can see some new NG steam in the US in 10-15 years. I'd pitch in some bucks.