Okay, There are two chars, both have 50% DR. Char [a] has level 20 Thorns (1010% damage return).
Char hits char [a] for 1000 damage (all physical), minus pvp penalty and 50% DR this becomes 83.33 damage.
Now Thorns returns this damage at 1/10th the listed value to char , so 1010% becomes 101%, which equals 167.5 damage.
Then the 167.5 damage minus pvp penalty and 50% DR this becomes 13.95 damage, the final damage returned.
So char [a] suffered 83.33 damage and char suffered 13.95 damage.
As you can see you would have to have roughly 6 times the life of the other char attacking you to equal the damage caused to you, with the damage returned.