Depends on:
- if you also include costs before you leave - which is a high percentage when you take into account the buying of your bike/car, all the equipment and stuff you buy for the trip, insurance, camera etc etc
- if you include the costs at the end of the trip - if your vehicle won't pass the safety test when you get home without spending 2,000 Euros do you include that?
- and the most obvious, what you do each day ;-) Sitting in the jungle in Gabon all day relaxing and walking around will cost you just the food you eat. But there is also a day charge for each country made up of the cost of visa/insurance/vehicle tax divide by the number of days.
The longer you travel then usually the cheaper it is, as you spend a lot more days hanging out. One day of driving in my landy and buying 3 meals could easily cost me 70 euros on fuel and food etc in Namibia for example. The next day spent sitting in a campsite would cost me 10 euros on camping fee and food i cook myself.
So you have basic costs we all face - visas, road tax, fuel, food from the market etc. Then on top of that is how you travel, hotels, game parks, restaurants, 20

s every night, mobile sim cards in every country.
The faster you travel, the more you will spend. I always try to spend my first day or two in each country by just staying put, learning some of the language, seeing what things cost etc i.e.: basic country research. This usually means I save money as I go as I have an idea of what things should cost etc.
Over 3 years I usually spent between £500 and £1,000 per month depending on various things as above. This was driving a diesel landy costing 10 litres per 100km on average and almost always camping (99% of nights).