The xp goes down for a given region as you level. Not sure how you didn't notice this.

Also, the experience gained is capped by your level, so if you wanted to refine your plan, and say, look for what the maximum experience that could be is, that would indeed be quite a project.

I think one person managed to get "The Explorer" title by not killing anything since level 1. I think he got to about level 6 by the time he completed it (although that would require some multiboxing or some dedicated friends to help you out).

With my toons in Northrend, as soon as I hit 77 I generally explore all the remaining grayed zones for xp - you can easily earn 400-500k xp using that medium (and get easy achievements points If you like them).
Of course It needs an epic mount for the best ratio xp/time but doing that, I can earn a few more gold @ 80 by questing :]
You can also earn a few xp in Outland if not done yet, but Eastern Kingdom / Kalimdor is not worthing it xp-wise and requires a lot more of time.

I definitely have comparison data for you. Recently one of our junior members, a level 22 mage was heartstoning to Westfall (I forget from where but it was in the Eastern Continent) She wound up in the Twisting Nether and sprit-resed at CosmoWrench. We escorted her to Shattrah (after a few deaths escorting a walking lowbie through Blade's Edge was particularly painful) and she had gained about 200 xp per new area where I had gained 1000 xp discovering them at level.

as soon as i hit 77 i explored the rest of northrend all of icecrown, storm peaks, basin, and most of zul'drak netted me 234K xp this of course included the "explores" that don't show new map area but you get experience so your mileage may vary