Instructables user rocket had a broken zipper on an otherwise-fine pair of jeans, and by cutting out the zipper, sewing two buttons and cutting and stitching button holes has converted his jeans to a button fly. Consider making this modification to any jeans with a broken zipper.
This hack will take around 30 minutes and a dollar's worth of sewing supplies -- if you have a sewing machine you can speed up the time stitching the button holes, but this is not a requirement.
Full step-by-step directions can be found at the source link below.
jean zipper to button-fly [Instructables]