I never found a good rundown of the cost required to make them when I decided to have a go at it, so I figured I'd track data while I went at it. I've sold three 70L/poison bokuto blanks in the last 6 months at just under 800m each.
In materials, it costs around 200mil gold to craft a 70 lightning/poison bokuto blank IF you farm your own runics, otherwise it's going to cost around 900m if you purchase them at 1m per (good luck finding a bulk supplier). In time, the crafting part alone takes roughly 17.5 hours of non-stop crafting and reforging with breakneck efficiency (roughly 450 reforges per hour). Farming 900 ash runics takes weeks of tedious bribing and filling BODs, but you get your runics for around 200k in gold/boards instead of 1m per.
The lightning/poison bok isn't the only thing you get from this, but the only other thing worth selling is a 70 lightning/cold bok blank which goes for about 400m and on average you should get 1 of these per poison bok. You might be able to get sub 100m on any of the other spell/resist combinations if you're lucky. Everyone wants either 100% cold (after enhance) or 70 poison/30 cold.
TL;DR It takes roughly 900 ash runic saws, 18ish hours of crafting/reforging, and several weeks of BOD bribing and filling to produce one 70lightning/70poison bok blank worth around 800m and one 70lightning/70cold blank worth around 400m.