With such an open ended question you will get figures branded around all over the place. Just as some folks can live in a house of 3000sq ft with utility bills less than half those of a similar family living in a 1200sq ft. Some folks are more conscious than others about finding ways to conserve their spending - some eat out all the time and have to have a cafe latte every day and others eat at home and pack lunch and drinks with them to reduce their expenses.
Whilst there is RV Sue and her canine crew, and many others over the years I've briefly read about that seem to enjoy life very frugally around $1000 to $1500 a month or less, there are many others that will spend up to even 5 figures a month average. It's how you intend to spend your time for those 6 months - moving a lot and eating a lot of fuel? boon docking with next to zero CG fees? Activities/hobbies/interests of little cost or expensive admissions? Many many things will come into play.
There are lots of folks that discuss budgets such as Wheelingit.us Technomadia.com GoneWithTheWynns.com, Roadslesstraveled.us Pastorscott.com (http://pastorscott.com/travel/2014-end-of-year-expenses-report/) Travels with Kevin and Ruth - Canadian couple from Ontario that CG host in Sask' in the summer months and travel during the winter to warmer climes. Camperchronicles.com is a couple that have been working on the road about a year or so.
There are lots of others but these kind souls that have shared, if you read a bit about them to understand how they roll should give you a bit of an idea to formulate your own budget albeit these are FTers.
Bottom line is there are folks that travel very frugally for $1000/1200 a month moving very little, there are a lot in the $2500 to $4000 range and then there are others that would find it hard to operate under $6000 a month. Just as in SnB living everyone has a different level of spending and needs to satisfy.
Appreciate that in the ideal world you would have wanted to have a firm or small variable budget suggestion from responders here but it truly is impossible to hone in within an acceptable range.
You truly won't get a true feel for what it costs for you personally until you are out there and monitoring your costs in the various categories for a few months. You will then adjust based on what you can afford going forward. Some that continue to turn a blind eye we have read end up coming off the road due to not being able to afford it = everyone has a different way of living/travelling.
Happy travels.