If they win, then they get their money back plus the damages. If they lose, then the defendants can pay their legal costs from those $1000000 (and the reminder goes back to the patent owner).
If you have a patent worth $1M, it is reasonable that you can finance a $1M bond.
If you have a patent you believe (and sue for) $400M, it is less likely that you'll be able to finance that bond unless you are Microsoft.
So, it needs to work out to a percentage (1%-5%) of what you sue - but probably something like "and you lose at least half of that even if you pull it back the next day".