What's your current monthly page views and unique users?
Although you'll be charging a flat rate for a fixed period of time, you should still consider what the effective CPM rate will be.
For example, ad sense has eCPM around $1 for some blogs. Social network sites probably have eCPM less than $1, while premium publishers such as NY Times will command $20, $30, $40 or even $50 eCPM.
Your site would fall somewhere in between. One way is to do a quick projection of your total page views and unique users for the next few months. Estimate a CPM rate based on comparable site. And then figure out your flat rate.
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