Personally, my rule is 2cpm for coach, 2cpm for hotels and 5cpm for biz/first (should be able to do much better). Otherwise, not worth it.

One thing that's not immediately obvious is also consider how many miles you'd EARN if you paid cash for the ticket and factor that into your cpm analysis. For instance if you want to fly IAD-HNL, that's about 4800 miles each way--9600 r/t. So if you value miles at 2cpm, that's nearly $200 of value back when purchasing the ticket in cash. So I'd use that new value of (cash price) - $192 = actual ticket price when calculating cpm.