In real life it's a lot harder to grow/ get hold of drugs and not as many people do, so that's why it's "easier" to make ammounts of money. Because the growing of drugs is rather risky people tend not to do it on the masses, rather import it from countrys where growing certain drugs is legal. This makes the demand for drugs increase rapidly, meaning more people buy it and "drug dealers" can up their prices, and have a huge market to sell it on. Where as in Argonath drugs don't give enough effects to make them worth buying, the only reason you'd buy a large quallity of drugs is that you could hopefully find someone that'd buy it at an even higher price than the price you paid. and it's also a lot easier to grow drugs, meaning a large majorty can sell these drugs with ease, making the market smaller and in less of a need.