Selling brownies1 on the street is an illegal way to make money, much more than that which can be earned through conservatively “legitimate” activities like selling art, music, etc.. Money earned is based upon an individual's charisma, persuasion, intelligence, street sense and business… and how illegal they are. More illegal = more money.
It raises street sense, business and persuasion, and all three skills equally contribute $3-18 (for a total of $9-54). The value is doubled on C drug laws, and quadrupled in C+ drug laws. If drug laws aren't enforced, the income is quartered (or reduced to 1/8th.)
The odds of being spotted depend on street sense and the drug laws (where it is not enforced if L or L+). If caught and tried, it has a "scaryness"-rating equal to arson, racketeering, escaping prison and assisting escape. Minimum sentence is 3 months for drug laws C+ to M; during L and L+ it carries no prison sentence.
Note: In SVN 490-current (739). the C+ drug laws seem to have a lower risk of getting caught than with C drug laws. However, you are instead dealing with death squads instead of regular police.