There is a nifty double roundabout between Brazil and Guyana where you enter it one side of the road and when you exit you're on the other side of the road.
0: http://basementgeographer.com/crossing-from-right-hand-drive...
I spent 2 years on the Pan-American from Alaska to Argentina because I wanted to. There is no rush.
I expect Africa to push closer to $20k/year.
If you're interested in this kind of travel, checkout http://wikioverland.org - it contains all the logistical information you need to make a trip like this. (it's packed for the Pan-American right now, and soon to be loaded with Africa too)
[1] Full breakdown: http://theroadchoseme.com/the-price-of-adventure