You can buy a receiver hitch for about $150. Then get a cargo carrier that mounts in the receiver hitch like this one:
Amazon.com: TMS T-CCR1948KB Heavy Duty Hitch Mounted Cargo Basket Luggage Carrier Hauler, 2-Inch: Automotive
For $40. (I've never used this particular carrier, but it looks reasonably well made, especially for the price)
Then get some waterproof totes and strap them in when you go. That's well within your $500 budget and keeps your luggage in your wake as well as down low so it's easier to get into it. It does make the rear hatch harder to use though...