For most .25 to 1.20 size planes, a 15lbs digital fish scale is the most cost effective. You want to buy a scale with a full scale just large enough to cover the heaviest plane you'd want to weigh, as these things have only 2-5% accuracy. In any case, you could always weigh the fuse and wing separately.

I augment the fish scale with a 5lbs digital postal scale for smaller thing requiring much better precision, like engines and parts.