Stocks and drugs are two totally different animals.  If we had a stock market based upon a similar system as the drug prices, then you would just sit and wait for your payday.  Thats not how the stock market works.  Stocks lose money, a very very real risk in losing money.

The drug system now, you can not lose money, you literally have to intentionally try to lose money to actually lose money.

By adding a stock system based up on drug prices it would be the same as giving people unlimited units.

A real stock system needs to mimic real life stocks, meaning you can research it to make educated guesses, and you can make tons, and lose tons.  The problem is coming up with a system that cant be gamed by unscrupulous users.  Real time feeds are way out of our budget, slightly delayed actual stock feeds are affordable, but able to be gamed.  The smallest of delays means instant profit for anyone who can submit  sytem updates already knowing what the new values will be.  If we obfuscate everything, then you lose the research aspect of it, then it would be just driven by luck which strays us further from stocks.

Its not a bad idea, its just not entirely feasible in a safe way that cant be exploited easily.