I probably shouldn’t even comment on this thread since I haven’t made a hive scale yet; but I have done a little research. With regards to load cells on each corner, that becomes very expensive without the use of mechanical levers. For example, if you wanted a scale that could measure up to 400lbs, then each corner load cell would need to have a range of at least 100lbs. You would want to over engineer that since hive weight could be more on one corner than another. So say 125lbs load cells.
Not saying this is the best place to buy the things, but ebay prices for load cells having a range of around 100lbs typically run around $100 Each. Since you would need 4, that would be $400 just for the cells.
On the other hand if you use mechanical levers with the load cells (like most bathroom scales do) you can cut the costs of the load cells dramatically. If you implement a 10 to 1 lever, then each load cell only needs a range of 10 to 12lbs. Those run about $8 each on ebay. 4 x $8 = $32 plus the cost of the mechanical levers.
Then the problem becomes finding the room under the bottom board to implement four 10 to 1 levers and the cost of making the mechanical levers. If you’ve ever taken apart typical bathroom scales, they have quite the elaborate mechanical lever system in there.
If I were going to try to make a set of hive scales using load cells (maybe someday), my thinking was to buy two 10lbs load cells for about $8 each and build 2 mechanical levers (front and back) with a 20 to 1 ratio. However who knows, by the time I ever get around to that project, the 100 lbs load cells might come down in price to $10 and then I could skip the mechanical lever mess altogether.