Correct, however it's actually quite hard to obtain advertisers every month. The site in question doesn't have strong traffic and i'm guessing it's not a community (like this one) so there won't be many returning visitors.
I personally wouldn't be interested advertising on a shared rotator with 5 other banners for $35/month with that traffic. Talkfreelance has a significant amount more, we are close to 2,000/day now. Also, we only charge $35/month for the top banners and $75/month for the skyscrapers now
If I could get more money out of this website then believe me I would! Last month I managed to sell all the spots (header) whilst the month before when I was selling by the impression, I only shifted 10,000 impressions worth alot less.
Impressions work best on very large websites where companies will advertise and will pay anything! Sitepoint/webhostingtalk etc. On a small scale site like talkfreelance I strongly believe monthly rotating banners are best, on an even smaller scale site like we're discussing a static banner is probably the ideal solution, at least until traffic picks up!