Economics cannot truly answer this question because this is a normative issue. It is an issue based on values rather than on economic principles.
Legalizing drugs would, according to economics, make for a more efficient market in drugs. However, one likely impact of this would be to actually decrease the cost of drugs. There would, for example, no longer be a premium paid to dealers who have to run the risk of going to prison or of being killed in gang wars. Economically speaking, this is a good thing in that the same product would be available at a lower price.
However, this is an area that really goes outside of economics. Economic thinking cannot decide which products are good or bad. It can only talk about how best to make those products cheaper and of higher quality. So legalization would help the market for drugs become more efficient, but we still have to ask whether we want the market for drugs to be efficient or if we want it not to exist at all.
My question was not exactly about and open market for drugs in which market itself could regulate it. Or the impact in the price of drugs. but in a market which such as illegal substance would be regulated by the government. And how could positively or negatively impact government spending. And of course besides the fact of the moral issue. This actually help for class discussion.