I often program a slight taper to I.D. threads to counteract any flexing of the thread tool.
6" long 4140 Hex parts, I bet material is pricey. Seems like the price of 4140 hex has quadrupled over the last 10 years and then some.
Hex? Oh yeah, that is not a cheap bit of material.
It seems most materials have gone up in price; though, the price of scrap [anything] is lower than I have seen in the last 10 years.
I have made some 4" long bolts out of 4140 for the same customer for the last 12 years. Back on the first batch I had a local source I got the hex from for a little over $1 a pound. When it hit $4 a pound 6 years ago I just started making them out of round and milling the hex head.