The amount varies, but the last two places I've worked have spelled out their referral bonus amounts and terms explicitly.
Also, I want to make a nuanced but important point here. The employer in this particular case is explicitly kind of saying, "yeah, it might have kind of helped at the time, but you're not working here anymore." By that logic, every single person who ever gives notice should have their last paycheck docked if the time to cut the check occurs after they have left. After all, even though the paycheck might have originally helped them work there, they're not working there anymore so that doesn't matter. Why should an employer ever pay the last paycheck? You're not part of the team anymore and the paycheck is meant to motivate employees to work, that's why it exists. "Also, you dropped the database", or some made-up excuse.
I think we can all agree that people don't just work for money, but we can also all agree that every employer needs to pay the last paycheck, even if the time to pay it comes after the employee has left. To me, per my comment that you replied to, the fact that the database thing was brought up means the employer is trying to squirm out of settling a debt; rather than say that it wasn't a real debt.