This depends on a number of factors such as your driving style and the terrain you are crossing.
The Buhanka campervan averages 10-15L per 100km and has a total fuel capacity of 77L.
The Lada Niva averages 10L per 100km, and has a fuel capacity of 42L.
The Toyota Hilux averages 9-10L per 100km, and has a fuel capacity of 80L.
Petrol prices vary of course, but they average around €0,60-0,90 per liter.