Police Briefcase

For example, one quarter of a million dollars
I write a script and I was wondering if this makes sense. If an undercover cop who is investigating drug trafficking in the area and found a person with great amount of money, would that be enough probable cause to detain and take that person into custody?

Yes. They could arrest you on suspicion of money laundering, drug running, all they wanted the least plausible. And then, one could say they have received the money that the product "of crime, even if they can not support their charges, and then you have to prove where you got the money (in the case of large sums of money, you're guilty until proven innocent), or lose it forever. Good luck!

If they have no proof of where the money came from or why everything is cash then it would be the likely cause. (Suspicion of illegal activity)

Well, if your writing a screen play and then not really make a difference in the extent it is credible to the public.

In fact, I think it would need some sort of probable cause that it was stolen. They were more likely to watch you and see if they find something.

I do not carry around that kind of cash is illegal in itself, but it should be illegal to be stupid. I think if the guy is already a suspect and he was caught with that much cash after seeing something with someone to swap would be a reason to take him in and find out where the money came from (maybe?). One could not accuse him of anything, unless you could prove that he did something illegal to get it.

Yes if you try to go through the airport with 18,000 cash, you will miss your plane. they have to check where and how you met that amount of money.

Carries a large amount of money would be probable cause to detain you. They can keep you up to 72 hours (excluding weekends or holidays) before they are formally required to take charge of a crime. Even if you are released and not charged with a crime, they may still hold the money until you prove that the money owed to you and came from a lawful source.
Incidentally, no, you can not use the money to get you out of trouble.

No, having money is not a crime, he would have another reason to stop and confiscate his money.

It's actually a reasonable suspicion and not probable cause. With reasonable suspicion, the officer must also question the suspect, or consent to search his car to find some kind of probable cause that the suspect may make headlines. The beauty is that whatever it is, can easily be nothing to do with money, but to allow them to detain the suspect and question him about the money.

Then have your agent to find a pipe or a gun, or stacks of unpaid parking tickets, or a suspended license. Things like that.

I think this is a likely cause. They did it in "Running Scared." (Comedy)