The not-so-secret-secret is that they just buy more than they need and slang the rest to other gym rats. Those $400-$500 kits are no more than $200 max from a half decent source. Buy 5, 10, 20 or even more and you're paying pittens with the subsidy alone. Sell a few of those are full "retail" value and you break even. Sell more aggressively and you're banking money.
To do this though you have to have a lot of patience. Bodybuilders are fucking idiot losers and selling anything to them is a shit show. Most higher level competitors I know just stock way the fuck up, sell a few kits or some gear to newbies at twice the cost. If you can get away with paying for your drugs, you're set. But to be honest, because of how fucking lunatic most bodybuilders are the competitors I know at least turn to selling narcoticos. Much, much, muuuuuuch easier. Even a crackhead will be a more appreciated customer than a bodybuilder.