If I'm recalling correctly the only mention of 'horse's weight is "around half a ton" (1000 lbs) which is really early in the series and if I'm remembering it may have been by /u/Ctwelve during a chapter of Good Training. It's also been mentioned frequently that he max's out a pallet scale, which after some quick googling is around 5000 lbs (2.5 tons, link below).
I agree, some of the scales are mind boggling and ridiculous, but I think this is why Hambone rarely use actual numerical figures, just so we can figure it out ourselves, and keeping those figures straight would (as we can tell by the above) would be a job in itself.
https://www.amazon.com/5000LB-Durable-Floor-Pallet-Scale/dp/B007WXMFZG