Well, I began my project planning to use a 19" minimum monitor. Actually a television, for serveral reasons. For one, a 19" television is actually 19" on the diagonal, and they're way cheaper than computer monitors. Plus, for the purist, they're 15kHz, just like all classic arcade screens.
Ok, so I find a 19" TV, a the local flea market, in great shape for $35. No video input, so I'll have to add one -- most modern televisions will expose a CVBS signal between the tuner and display chips, so it was worth a shot.
But then, the very next week, I came across a 25" set at a yard sale. They wanted $50, I talked them down to $25... it seems the set had a problem -- it would mysteriously power on by itself. Not a problem for my setup, which will have its own power switch. And this one had the CVBS input, and has since checked out as being 100% working.
My point being, the TV sets are not too difficult to scrounge for cheap. You shouldn't settle for a tiny monitor.