Got myself a hand held electronic luggage scale to weigh my bike for something like 18 bucks, IIRC, from Bed and Bath. It seems to be accurate enough as it very very closely matche the weights I get from two digital bathroom scales I used to use to weigh my bikes through the weight subtraction method ( My weight with the bike minus my weight without).
There are scales from bike accessory companies out there, but they seem to be way overpriced for what they are.

Chombi