Here's a chart that sums up a very well thought out, 1997 peer-reviewed study on the nitrogen intake of carnivorous plants, with a focus on Nepenthes mirabilis, Cephalotus follicularis, and Darlingtonia californica.
I was able to access the journal through my university's database and have it saved if anyone would like to read the entire piece. Otherwise, if you try to find it through Google Scholar or JSTOR, you will have to pay $35 or thereabouts (hence why I will not be posting more than the chart).
As you can see, Cephalotus seems to be acquiring only about a quarter of its nitrogen through insects in the pitchers, leading us to believe that roughly 75% of its nitrogen intake, with a standard deviation of .06, is through the soil (particularly at a young age). Nepenthes mirabilis seems to be acquiring a much higher level of its nitrogen from insects, with roughly 40% nitrogen coming from the soil or perhaps from nitrogen fixing bacteria. I'm sure you can all read a chart, so this is probably redundant commentary.
Anyway, I'm sure many of you have already read this journal, but I found it interesting enough to share.
The Nitrogen Supply from Soils and Insects during Growth of the Pitcher Plants Nepenthes mirabilis, Cephalotus follicularis and Darlingtonia californica
Author(s): W. Schulze, E. D. Schulze, J. S. Pate, A. N. Gillison
Source: Oecologia, Vol. 112, No. 4 (1997), pp. 464-471