I have looked into it but it typically strikes me as totally insane in conversion math perspective. Have you seen the rate cards for teaching magazines, the NYT educational supplement, etc? Egads. And then I have to get them to read it AND get them to the website AND get them into the trial AND make the sale... and then I get $30.
You have your other variables tracked well enough that you should be able to see the difference pretty well and see if it's worthwhile.
If you go for the low hanging fruit first (a holiday) where the effect should be biggest you might get a nice surprise - or possibly not, but at least you'll know for sure.
You can probably halve the price on the rate card to get the real price you'll pay after negotiating a bit. But agreed, still seems expensive even after that.
I'll stick with my 5 cent CPC ads, thanks.