The holdups in Cops and Robbers are illegal ways to make money, or better to steal money.
You need to go to any 24/7, Ammunation, Bar, or restaurants in the server's active city, go in the shop, and type /holdup. This will instantly raise your wanted level by 7, so you will be a Felon. Then you have to wait about 10 seconds, while the clerk opens the register, and then you will get 2000$-6000$ per seconds, the amount tends to be high when you have a crowbar, and the register opening time decreases as well a couple seconds.
You have to pay attention, because cops can arrest you just by going to the shop and by pressing MMB.
To finish an holdup, when you get enough money or simply you know that cops are there, leave the cash zone, and get out quickly. The best part is that you can try to bribe police officers and get innocent if they accept the bribe, so you don't get arrested and lose your crowbar and your weapons.