I don't do CPM because I prefer results instead of the possible brand recognition, but CPM ranges usually somewhere between $2 and $3. Depends a lot on the site and its stats, the traffic (geo) source and such metrics though.
I'd have to agree with MasterA, $1 per 1k impressions is probably the highest I would go. However it'd depend on how much traffic my advertisement would get in the first place based on the websites reputation and overall success.
Depends on the site. Well-optimized and targeted ads might pay $5 CPM and get such great results that it's irrelevant, and I've seen "cheap" ads where people really do pay that much. Different kinds of ads cost more, for instance video ads. A better audience helps tremendously.
On the other hand, low-quality ads have low prices. I'm a forum admin, so I'd probably try to keep ad spending to a minimum, and at the moment (with only a dead forum) I haven't bought any ads personally.
Most of the CPM networks out there is too expensive. I saw you guys (and girls) saying $1, $2 and some even say $5. I spend only 0.05 USD (in Bitcoin) to advertise on a site which have Alexa rank of #11199 and more than 100 daily visitors - BitcoinAliens.
Although it might be a faucet but.. since I'm advertising a Bitcoin related site which is also a faucet.. I'm getting good results. Spend ~0.02 Bitcoin yesterday for 90K impression. The ~5 USD I spend return me with ~200 clicks and 50% increase in my members count.
It would entirely depend on the quality of the impressions. There's no point saying $1CPM is my Max because you might find paying $2/CPM to advertise on another site generates 3x the signups per 1000 views.
However: your risk does goes up as you pay a higher rate for advertising. I guess I probably wouldn't pay more than $3 /CPM based on my circumstances.
My max on paying per thousand impressions, is usually $2.50. I tend to not want to pay any higher than that, due to the possibility of not earning back what I spent on the impressions.