As others have said, it really depends on how far you travel, whether you use campsites, wildcamp, aires or a mixture, what your tastes are and when you go but we regularly spend six months or more travelling around Europe over the winter half of the year and our total costs come to around £1,000 per month. You can work it out by taking your normal expenditure on food/housekeeping (food is not so very different in cost but alcohol is a lot cheaper!), adding on the anticipated fuel usage based on your proposed itinery, adding in ferry costs and the entrance fees to any attractions you want to visit and then seeing whether or not you have enough left to stay on campsites or whether you need to wildcamp and use free aires. One thing, however, is sure - you will have a great time. Happy travels.