Depends on your jurisdiction. Simple possession of a small amount is releaseable on a summons where I'm from (just like a traffic ticket). So say he's being a major dick and doesn't sign the summons, he gets brought to jail, has to go through booking and will probably get released on an unsecure bond or possibly no bond (considering he could have been released on a summons). He'll have to come back for his arraignment hearing, unless he's an idiot and skips out on that. Then, it's jail time.You forgot to add on the legal fees. Even a mediocre lawyer is gonna set you back 2-$3,000 to work a simple possession case. And spending 24 hours locked up waiting to be arraigned isn't gonna make your day any better either.
Though if you're lucky enough to live in New York possession of 25gm or less is a violation. That's equivalent to a speeding ticket though the fine is much less. Unless of course the NYPD asks you to empty your pockets during a Terry stop (i.e. Stop and Frisk) and you pull your stuff out in which case it's "public display" which is in fact a misdemeanor.
And if the arresttee is poor, he doesn't have to pay a dime if he's eligible for a court-appointed attorney. And, he also has the option to represent himself. Why pay money for an attorney to plead guilty or no contest when you can do that for free on your own. It's a misdemeanor, not a felony.