Then prove him wrong by show the tax bracket in your country are high, why make it complicated? 
I get your point but this is just an example. It's not a real scenario. It's more like the case of one person seeing a "6" while the other person at the other end sees a "9". From each of their point of view, they're both correct, but they may be seeing different sides of the same thing.
Disagreement is normal, it's not even rude at all if other people's opinion are correct.
Even if someone else's opinion is wrong, it's not rude, because at the time, he may not know that he's wrong, unless he is rude about it.