Last time I checked with for example futurlec, they took .35 USD/cm² (2-L cheapo), so a 12x12 cm board will cost 50 USD. But a MiniITX would cost 101 USD. A doubling of the cost. And in the end that space isn't likely to be used but will be just a filler to accomedate screws.
It's way more economical to use a plain plastic/metal board of 17x17cm and cut out a square that is 5mm or such smaller than the minimig to allow for screws.
That way you could also use a minimig (144 cm²) in a 3,5" slot (15.24x10.16cm = 155 cm²) with minimal extra cost to fit. And such slots are available even in a 80386 chassi should anyone feel like it or a off the shelf external harddisc chassi.
