Are you saying that all Mac Pros are manufactured at this factory, but because the number of Mac Pros sold is less than other models, because they're sold to a certain target market, and (possibly) because they're made in the USA they're more expensive then other Macs like the iMac?
The US factory doesn't churn a lot out. I don't have the numbers, HOWEVER, because the output is very low, the price is high. Very high.
Also, some people (in Apple) like to brag that this factory churns out "custom-made" Mac Pros, hence, the price tags.
The factory exist, not (just) because of this, but also gives Apple a "tax benefit": Factory sits inside US territory and has a monthly output >1. This means Apple is entitled for the tax