Just charge accordingly for the projected extra time, fuel, and drive-time involved. They can be more profitable than really nice lawns if the lawns are actually good prospects for biweekly mowing. Just don't try to mow luxurious thick lawns like that.
But for a lawn that gets no fertilizer, irrigation, etc, it's worked well for me. I'm averaging about $75 to $80/man hour on-site vs about $60-$65 for the nicer weekly places. Since I do the work I'm more interested in revenue per unit of exertion than total dollars, period, as my limiting factor is fatigue, not time.