$200 = $60 dollars in commission
$400 = $120 dollars in commission
$600 = $180 dollars in commission
$700 = $210 dollars in commission
*since it was only $100 more (which is 1/2 of the $200) then you also cut the commission in half to $30 then add those $30 to the $180 and you get $210
hope this helps :)