I've found that Arizona is one of the cities where it's actually better to buy the tickets in advance rather than waiting for right before gameday, which is what I'd normally suggest you do pretty much anywhere else. The past 3 years, the prices for that game have either stayed the same all the way until kickoff, or rose in price as the game has drawn closer.
Right now, there are tickets available for as low as $108 a pop on Stubhub for that game. If both teams start off hot, or are still in the race for the NFC West come kickoff (highly likely, at this point in time), you're probably looking at cheapest ticket being anywhere from $150-200, probably in that $175 range.
Keep in mind that tons of Seahawks fans make the trip from Seattle to Glendale for that game, so tickets are always going to be in high demand for that one, even if the Cards stink (which although ideal, probably won't happen just yet, but who knows?)
In this case, I'd err on the side of caution and peace of mind, and buy the tickets now.