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I do pretty well with Adsense on my site and want to remain constant with that. I Any general figure would be of great help, I am looking at a starting point for a 160x160 image ad. no tracking, no cookies, no ppc
Assuming that your traffic is steady you can just pick a value that sounds reasonable. Not knowing your site I'll just pick $10 CPM so the math is easy. $10 CPM at 40 thousand impressions comes to $400 a month in total inventory, and since you don't want to do tracking, you probably also don't want to worry about rotation, so there you go. $400 a month.
For a very targeted, local site you could potentially look at $20-30 CPM or higher, depends on how targeted the site is. The more targeted the site, the more it's worth.
Does that help?