500 Gbps DDoS amplification attack? No. Not for < $400.
Besides, are you saying you're willing to spend less than $400 to stop a 500 Gbps DDoS attack?
ASA 5505 can handle 10gbs?
No, not even close.
You really need to think about it: You want to stop a DDoS on your 10 Gbps link and you're going to spend only <$400.
If you have the funds to pay for a dedicated 10 Gbps link then a firewall that will support this speed is worth more than $400.