Look at the price of the powersupply and the cdrom adapter that comes with many mini-itx cases.
If you already have those items, nevermind; but if you still need to buy those items, then some of the cases become more attractive.
BTW you can get a real 12vcd in powersupply up to about 70watts.
So you can pipe 5 amps of 12v from your car (regulated would probably be better) right to the computer.
Or you could buy a larger powersupply that runs off 110vac and have to run an inverter.
If you use a laptop cdrom (with adapter) you can power your computer off that 70watts.