As a publisher, I can shed some light on this. The sites you are calling are just throwing out that quote. Its almost their "rack" rate. Some advertisers come out of the blue and just don't blink at paying off the rate card....to this day this amazes me.
Our remnant inventory migrates from $1.17 to $2.05 depending on the site. Then we use vertical networks to fill the mid range, which is targeted buys from $2.50 - $6 (showing a BMW ad on an auto site is what I mean by targeted). Then the large agencies step in for CPG, Entertainment, Auto and Health branding campaigns that go from $5.50 - $12.
The lower the rate, then the less control you have. Its often below the fold, uncapped, no geo-target, no day target..etc..etc. As the CPM moves up, then those doors open since the campaign needs to be managed more. But even at the remnant rate, if you just need a broad demo ( male or female) or an age group, you can get in there most of the time, you just need to step up with a $2.5K - $5K test buy.
I have 40MM impressions on an entertainment site that needs $3.50 for a guaranteed fill, but I can take anything north of $2.25 on a best case for remnant. I also have 14MM impression on an outdoor network ( hikers, climbers, extreme sports, action sports) that can take $1.50 - $2.25 for a guarantee.
If anyone wants to test, PM me.