I've priced a 20,000km trip at 1000$CDN in fuel alone. This would be from Sudbury to Laredo, then around the cost of Mexico, Belize, Honduras, (everywhere else no Carnet is needed) Guatemala, Mexico, Baja, Laredo, Sudbury.
Based on 1.50US/Gal in the States, 2$US/Gal in Belize, 75Cents/Litre Canadian in Canada and 75Cents/litre US everywhere else. I hope to get about 6 litres/100km. All of my prices are a little high just to cover extra gas spent tooting around towns after arrival at the end of each day.
What is the current price in Mexico? Two years ago the price was quite good and we were filling our 60 litre tank in a van for about 300 pesos.