As a not-yet-employed, interested-in-someday-freelancer (freelancing? ... youknowwhatImean), I've found this revealing.
Would the rate of pay vary if there was research work (or design work) involved? For instance, would you pay more for "crunch" (particularly "good crunch" that had been playtested)? Or for "historically accurate 15th century" content?
I can't speak for anyone else, but yes - ENP's per-word rate may well vary depending on the complexity of the article.
Generally, we tend to use rpgFREELANCER.com these days, and let prospective freelancers submit their own prices. Then we choose one - and to date we have *never* chosen the cheapest one.