You have some pretty good advice already, but I'll point out the economic side: If magic really does work well, then a thriving black magic market would arise, and costs would be exorbitant. Every so often there would be scandals regarding the high-ranking citizens, and politicians or rival guildmasters would be constantly accusing one another of collusion with the sorcerers.
Out of curiosity, why is magic illegal? Because this could also give rise to a situation much more like Medieval Europe, where magic's illegality was secondary to its powerful social stigma - sorcerers were viewed as pagans and diabolists. There was no real black market for magic (that I'm aware of!) in medieval Europe, but there was a sort of monopoly on the practice, controlled by the Church. And there were also certain grey areas, such as astrology and alchemy. The main theme there is that the more pagan or satanic a form of magic appeared, the less it was accepted anywhere near the mainstream. It was only wisewomen and hedge wizards at the fringes of society who got away with blatantly unChristian practices, and the nobles who were powerful enough not to fear Church edicts and collected magical books and talismans anyway.