How much will you have? Seriously, that will determine what you can spend. If you won't have much, then you will probably spend more time in each spot, which cuts down on park costs, fuel costs, etc.
Like Engineer Mike, we budget for around $5K per year for maintenance and repair (we have an orphan Alpine). The last two years (knock on wood) our repairs have been considerably less - we got most everything done that first year. If you get a used rig (which is usually a good idea) put at least 10% aside (or more) for repairs/upgrades.
We keep our park costs down because we have a membership in WHR plus some affiliates. For us it works because we use the systems at least 2 weeks out of every month. We like to move - 2 weeks or so, then down the road.
We know people that do well on $2000/month while others are stretched at $6000/month. It really depends upon how you live your life. A good way to get an idea of what costs are and how people live is to read the blogs that a lot of fulltimers have online. Also, check out the Escapees forum (
www.escapess.com) as there are more fulltimers there than anywhere else that I've seen.
Barb