But with drugs I don't think it's the dealers we have to worry about. The dealers are the little fish. It's the financiers, the money launderers, the ones who profit. They -- if they are smart -- never touch the stuff. They invest in it like they'd invest in any other venture.
My suspicion is that a captured DEA would behave not as an actual facilitator of drug trafficking but as a selective turf war tool to take politically unconnected competitors' drugs off the street. All you need there is enough influence to ensure that the DEA targets the other guys, not your bread and butter. That also helps to support the price of drugs, increasing your margins.
Actually it could be even simpler than that. If you have enough spies in the DEA you could be one ahead of them, have intel about how they conduct searches, etc. Then the DEA would act for you like a passive turf enforcement army. With your advantage you could fly under the radar, allowing the DEA to mop up your competition and prop up your margins by creating scarcity. Spying on an organization is orders of magnitude easier than influencing it, and requires the compromise of lower-level personnel. There's far more field agents and mid-level paper pushers than there are higher-up agenda setters and they make less money, making them easier targets to flip.
This is why the drug war is so bad for democracy and why prohibition encourages corruption. Put simply it creates a massive financial incentive for every kind of political corruption imaginable, and in so doing begets a "corruption infrastructure" that can be used for other kinds of crimes.