I actually had a caregiver until recently. It worked out really well.

You are absolutely right that criminal drug dealers have no reason to sell their product for cheaper.

Criminal drug dealers could never sell as much product as legal ones however.
Criminals cannot advertise. They cannot have a store front. They cannot take in a new crowd simply by seeing ID or paperwork. They cannot, because they fear getting busted, keeping their clientele much much smaller.
Criminals cannot hire from a large list of employees, only those they trust. Hiring only criminals does not usually result in a large or well run corporation.

If you look at the amount of legal drugs sold by drug companies producing advil, perkoset, viagra, tobacco, alchohol etc it's very obvious how much larger that industry is than the illegal side.