Tractor Supply had a great little electronic scale on sale this summer. Not sure if they are still at your local store, but check around the bargain bins by the checkout. Price was between $5-10, IIRC. It’s small (hand-held) and your hay has to be contained in a net, or just pick a bale up by the strings, it will weigh anything up to 100lbs.
I did a study this summer on how much my horses consume, using that scale to weigh their nets before and after each feeding (2x daily). The “picky eater” ate about 22lbs on average, while the good-doers ate 25-30lbs daily. All fed free-choice O/A from small hole haynets.