I can only imagine that part of it is because I haven't provided a tech drawing (I wanted to see the price without threads) and part of it because of the filleted internal grooves, but even in the ideal case it'll probably still be >$20 more than the threadless 3D printing option. Or maybe this part is harder to make in aluminium instead of brass?
It sure sounds like the second attempt is going to be a 3D print, though I need to be able to avoid the $30 thread cost without the nozzle / heatbreaks leaking. I might just see if I can 3D print the bottoming M6 threads and then all I'd have to do is simply tap 3 M3 holes. I might model in 2 holes for each cartridge so that there's a spare one if I have trouble tapping the first one.
Additionally, I'd need to change the geometry of the internal channels to ensure that they actually print properly. 1mm diamond square, anyone? The height of the block is likely to be near 30mm but it's better to be conservative when I'm not a company with a fat R+D budget.