It is done. I don't know what would make it cheaper than using drop bars, though.
Seems like most of them as use flat bars are also singlespeeders. Maybe there's some sort of affinity between SS 29ers and the SS cross. Also, there is sometimes an incompatibility between mtn shifters and road drive train, so most people just stick with road bars.
Thomas Frischknecht nabbed a second place in the 1997 World Championships using flat bars. UCI subsequently banned the use of flat bars in cross, but they are fine for almost all local races (provided you leave the bar-ends at home). [But I've read stories about overzealous butthead organizers doing weird things this year, so be sure to check.]
A lot of guys spend 95% of the time in the drops, so why not just use flat bars and have better control of the brakes? If you grab the left drop when you shoulder the bike, you have to figure out a different place to grab, no biggy.