Pretty risky, but I like your thinking.
Here's a bit of info on how it works.
$500 @$67 returns $33,500, IF successful. It returns zero otherwise.
The odds for the premiership winner market will be suspended after the start of the first match in a round, and don't come up again until after the completion of the round. For any team that plays that weekend their odds will probably change, in Souths present predicament our odds will probably change dramatically. If the Roosters win you'll see $200+ for us. But our season will be over. If we have a narrow win, we'll probably come in to possibly as low as $34 (current price for raiders, panthers and dragons) but probably a bit higher. A big win we'll possibly drop into $20's.
A couple of other things to consider.
I'm unsure as to how you were planning on placing this bet. If you were going to open an online account, I'd consider Crownbet. They're paying $81 at the moment. IF your bet was successful, your return would be $40,500, not $33,500. Odds comparison website :
https://www.odds.com.au/rugby-league/nrl-futures/2017-grand-final-winner/
If you would like to mitigate your risk somewhat, you might consider placing the bet with someone that has an early Cashout option. Here's what I might do, I'll use $200 as an example. I would place 2 $100 bets, instead of 1 $200 bet. If I got on at $67 and we beat the Roosters and the Cowboys, we might come in to say $21. Because of the change in odds I'm now being offered a $300 cashout for each $100 bet. I have the option of cashing one out, banking my original $200 + $100 profit, and then riding the other bet home knowing you're playing with their money. Or I can let both bets ride if I'm feeling confident.
Hope this helps. Please gamble responsibly..