In reply to SarahElizabethMoran: Decide how scummy you are going to be (you sound pretty scummy if you are planning on bivvying a lot)
I would say this:
1) Decide where (approximately) you want to go. Use google maps to find the distances (remember to click the "no tollroads" box) and use this millage together with your vehicles fuel economy to calculate fuel cost.
2) Work out about how much you spend on food every month at home. Add this onto the fuel cost.
3) Add another 200 euros for miscillaneous stuff (there will always be the odd backery luxury or night in a bunkhouse when it is pouring rain) and as a safety net.
Total is what you need.
You need to add more if you will be camping in a site where you have to pay.
Main thing is to look at fuel and distance. Food can be a bit cheaper or more expensive than in the UK if you use supermarkets abroad but even a simple ring round S Europe to Lithuania and back round N Europe is going to be 5000 miles or more and that is 600 quid in a car with good fuel economy.