I think if I remember right Magnesium Sulphate is 9.8% magnesium by weight... 10%. Perhaps someone with a scale can measure it for you.
Most people only absorb between 25% and 75% of the magnesium from a whole food dependent upon their current health etc. Considering that in the last 100 years or so magnesium levels in foods have dropped one would consider supplementing in some way.
Magnesium absorption from Epsom Salt would most likely be much less than absorption from a whole food... that is where the skin comes in; transdermal absorption may be your best bet if you are looking to increase magnesium levels in the body over and above what you may get through foods.