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What would be a justifiable price to charge on a CPM basis? Some sites charge $10 CPM, some $1... I've even seen up to $60 on a very large established website.
How much "more" is depends on a number of things. Consider the products/services that your advertisers are most likely to be selling. Products and services with large profit margins can afford and expect to spend more on advertising. One example is the shared web hosting market, where a company can expect to spend ten times as much money to gain a customer than they spend actually serving that customer.
Advertisers are only going to stick with you if you're helping them make money, so demanding $60 for a thousand impressions might get them the first week, but you'll lose them after that if they don't see a return. It sounds like you're just starting off with the CPM, so try not to scare off advertisers. Take queue from other publishers in your niche and start off at their rates, then increase or decrease from there depending on the response you get.