Advantage is like around +4.5 bonus to hit, so u hit with advantage and enemy hit u with disadvantage, it is like +9 net, your comment is wrong.
Speaking about arithmetic mean (average) there is 3.3 difference. Standard roll has mean 10.5 while advantage or disadvantage has 13.8 resp. 7.2. Median for d20 roll is 10 or 11, whille median for advantage is 15 and disadvantage has 6.
A much more mathematically inclined friend of mine showed me the full breakdown, but it's actually slightly more than that because Critical hits and Nat 1s come into play to skew the averages a bit more.
Additionally, the rules of 5e literally make Advantage/Disadvantage equal to a +-5, at least in terms of calculating Passive skill checks. PHB page 175, "If the character has advantage on the check, add 5. For disadvantage, subtract 5"
