I buy 10 bitcoins at $10 each, cost is $100 plus $0.65 commission.
I sell 10 bitcoins at $10.12 each, I get $101.20 and pay $.55 commission.
It is not reasonable that for a larger transaction the commission is lower, it should be the other way around so there is a mistake here?
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If f is the fee, B is the buying price and S is the selling price then break even is found when S/B = (1+f)/(1-f) since:
Our income = -B*(1+f) + S*(1-f) > 0 => S/B > (1+f)/(1-f)
In the case 0.003 and 0.0065 we get 1.00601805 and 1.01308505.
In other words, if you buy at $10, you sell at $10*1.01308505 or roughly $10.13.