By knowing what you are doing! And who said you are "supposed" to make money? There are several guys who do rebuilds on here reasonably, lilbill advertises $25 a hole plus parts. The guide is there, I posted a link, do the sweat get the bux, no short cuts like buying junk carbs cause you don't know any better.
$300 isn't insane, those are beautiful carbs, his labor rate is cheap for the quality you get. Learn to do it yourself and you can rake in those "insane" $300 carb bucks. How can you call yourself a shop if you don't even know a LITTLE about carbs????? Sorry but something here is not adding up, please do tell us more about what you are trying to do?