the computer monitors that can display 15khz signals, like the nec multisync II, a common one, are generally really cheap since it;s old stuff, but you would end up with a picture that looks very similar to mame, except slightly fuzzy compared to modern monitors, plus I think the nec ones are only 14 or 15"
...I'm not entirely sure if this is correct, but I think it's possible to take a tv apart and feed the r g b sync & ground right into it, bypassing the tuner and stuff for dealing with composite video / etc.. it would take a bit of looking around at the circuits inside though. although even if thats true it's much easier to just get an arcade monitor since it would be ready for mounting easily and good to go.