I suppose you could make a DIY weight sensor using springs and position sensors (potentiometer etc) but for beehives I guess you want the whole thing to be solidly mounted and not move around, which might rule out that kind of approach. If you fancy making and calibrating your own load cell, you can buy a strain gauge for just a few quid. Then you have to figure out how to mount it, read it, calibrate it, compensate for temperature drift and so on. It only needs time and effort rather than money, but I don't know how that sort of thing appeals to you.