From detroit, airfare is between a little and a lot more this year compared to last year. I bought last year's President's Week tickets right around this time (Sept 04), and paid just a bit under $200 RT for direct flights on Spirit. This year, the chepeast direct flight on a regular carrier is almost $300 (airtran), and Spirit and Northwest are $400 and $500 per person, respectively. Even connecting flights are $300 and up, round trip.
I bought this year's tickets a bit earlier, in August, and got direct flights for the exact times/days I wanted for $280 pp. I could have gotten that down to $225 for taking less desirable times, but still on direct flights. In August, I felt like I might have been paying more than I necessarily had to, but wanted to limit my risk. Now that it's September, I'm glad I did.
If I were a betting man, I'd bet that prices may come down a little bit---$400 just seems way too high, even accounting for airlines deciding that they need to make money in the face of rising fuel prices, and I can't believe those planes are already full. But, I don't know for sure!
Keep an eye on the prices, and jump as soon as you see a deal you can live with. Don't wait long after SW releases flights---SW often doesn't discount holiday weekend flights nearly as much as other times, but it will probably never be cheaper than right then.
Finally (and I hate to say this, but...) you might want to have an upper limit on round trip price beyond which you won't go.