I just feel I "must have" one of these! :-D
I don't think the demand will be less than 10 pieces... but would it really be enjoyable to make any more than 5-10 pieces?!? Also, unless the builder uses CNC operated machinery, and does a much longer production run (100 pieces or more), I guess there will be no discernable decrease in production cost.
When this discussion started at the buntbahn forum, I mailed Mr Regner, suggesting he pick up production of a steam waterpump. However, he doesen't like writing mails, so I guess I will call him up and ask what he thinks of the idea. He's more of an action man, so so I would not be surprised if he allready is constructing a steam water pump ;-)
As to cost, in my view / guess, the pump is technically comparable to Regners smallest twin ossilating ship machines, with a list prices of Euro 200 (=USD 315). One may have to add the cost of a lubricator, adding between 12 to 40 Euros (=18-65 USD).
Although I will presumably not be a cheap accessory, I still think it would add enough operating fun to justify the cost! After all, you would now operate a second steam engine at the same time, so that ought to give twice the fun?!?! :-D