In my case, in Peru, it was a fixed amount, $1200, which is the max that you're liable for if you take out all the coverages that Sixt provides. It doesn't mater whether you're renting for one hour or one month, it's still $1200, so it has nothing to do with the rate you're paying. (The coverages that lower it from the full value of the car down to $1200 may vary depending on the cost of your rental, but the uncovered $1200 doesn't change with the cost of your rental.)

My guess is that anything that's a round number (exactly 500 Euros or exactly 1200 USD in my case) is likely to be a flat amount, and not really related to the cost of the rental. So I'm not sure it's really correct that Sixt does 2-3 time the rental amount. That's probably just an average based on average rental prices, but the lower the rental price, the higher a flat fee is going to seem by comparison, of course.