Tom,
You have asked a very good question. As I am putting my own labor into the project, and have not kept track of how much that is, it is hard to estimate that aspect of cost. I can tell you this, however, the big ticket items are the wheel sets and trucks. I was able to find some original useable 24 inch diameter wheels for my project at a reasonable cost. Yet, these are just not available any more. Therefore, you would have to have them made by someone like Mckees Rock Forge. They cost about $600-800 each. Then, you have to have the truck frames, journals, springs etc. I made all of those parts for my car. The journal boxes I made from weldments, ala Durango & Silverton practice. They came out looking and working great. However, it is a LOT of work in that there is a lot of gringing and finish work to make them look like castings. If I were to make a bunch of these, I think I would definitely consider a patterns and have them cast. As for springs, they are out there from new manufacturers, such a Beale Springs. You can have all the springs for a couple thousand. As for the truck frames, make them yourself, as well as the patterns for spring pockets etc. It ain't that complicated.
As for rthe car body, the big expense there are the sills. Count on about $1500 for the timber. Consider using structural glue lams, as clear timber in sills will scare the hell out of you. Also, the window posts, rafters and carlines are made of ash, but the expense is not prohibitive.
If you make the parts yourself, as I did, all you have in it is the cost of the wood.
There are a number of castings that have to be made. I made all the patterns myself, but you have the foundry expenses. For instance, the link and pin couplers cost about $150 each to cast. But if, I had to pay someone to make the pattern, it would have been a LOT more.
Interestingly, the single most expensive parts I had to make were the window lifts and locks. Since I a replicating the Billmeyer & Smalls car, I made identical parts to the original. It makes me shiver to think of how much I paid for that because it was such detail work for many many parts that, frankly, I could not do. If I were just building a car, and not replicating one, I would get some stuff from my local furniture supplier and a much lower cost.
All said, by far the biggest expense in a car is your labor. The material isnt that bad if you shop around. However, the other item often in shor supply, is the will to see the project through. While each part may not be that hard to make, believe me, there are lots and lots of them.
I hope this gives you an idea of what it takes. Have at it!! You'll have a great time. Just be patient and persistent.
Dan Markoff