Given the time of year, it will likely be around that $500 amount, but you still may find a deal. I usually rent for work about 25-30 times per year, and here are a few tips I'd suggest:
1) Book something now directly from one of the agencies, then keep looking for lower rates through either the agency directly or the various travel sites. Reservations booked directly through National, Avis, Hertz, etc. can almost always be cancelled without penalty. Just ensure you don't accept a prepaid rate.
This allows you to have cost certainty, in case supply becomes scarse later, and when you likely find something cheaper, you can just cancel.
2) As ken mentioned, rent from an off airport location. There is a hefty concession recovery fee charged at all airport locations, and for Halifax, it's over 16%. This fee isn't charged at in-town locations, so on a $400 rental, it will cut $64 off the price.
3) Keep checking, both on Hotwire etc. and on the agency websites. Rental pricing is very demand driven, and if cars are available, they'll drop the price. Quite often, I'll check with National as I'm landing in Calgary, and get a one-day rental for $20, which is far cheaper and less stressful than taking a cab home, which I then drop by my office DT the next day. You could cut the rental cost in half if there's cars available a couple of days prior to rental.
4) As ken also said, if the price difference is huge, book a smaller vehicle, and ask for an upgrade at the counter. Depending on availability, they'll move you into something larger for $10 or $20 per day. Be warned, this is playing roulette. If they are fully booked, you could end up in the Corolla you booked as a midsize.
5) Check for corporate or specialty rates. There's always car rental codes in the Entertainment book and things like that that could save you a few bucks. Are you, or someone in the group, a Costco member? You can book rental cars through Costco Travel in the US,
www.costcotravel.com
Hope this helps. With a little bit of persistence, you should be able to find a decent rate. The only caveat is, I hope you are over 25. If not, prepare to be hosed.