Minimum oral lethal doses of boric acid in humans have been estimated from accidental poisonings to be in the range of 5-20 g for adults, 3-6 g for children and <5 g for infants. Source: http://npic.orst.edu/factsheets/archive/borictech.html#acute

A minimum of 56g of salt, taken all at once, will kill you. Source: https://www.timesknowledge.in/science/about-us-humans/what-is-the-lethal-dose-of-salt-1757-2.html

So then borax is approximately 5-10x more lethal than table salt.