Like most birders, I undertake most longer distance trips and days out as part of a car share to keep down petrol costs. Trying to work out a reasonably accurate cost per person when your brain’s fried after x hours driving and y hours birding (x + y > 12 for a decent day out) can be bit of a pain.
See http://www.fuel-economy.co.uk/calc.shtml for an online calculator that will do the trick. Just enter distance travelled, fuel economy and fuel cost, and you’re done. Simples!