I've been told this is a cheap way to fix small windshield cracks. Never tried it but sounds like it would work for the small spider sized and shaped cracks from small rock impacts.

This is basically what the glass repair kits sold at auto parts stores are. (They also include a suction cup with syringe, to vacuum any air bubbles out.)

The expensive way is superglue plus a little suction cup to evacuate the air, and a razor blade.