I agree with the above- there is no standard. You'll have to call around.
For instance, I know people live at Disney's Fort Wilderness campground. There are no weekly or monthly discounts and they pay something upwards of $150/night or more at times. When I looked at staying there for a month, I was looking at between $3,000-4,000.
Additionally, I explored (very briefly) what a monthly stay at Bluewater Key would be. In the peak season, they say they don't offer discounts. So you're looking anywhere from $150-200/night which would give you a monthly cost of $4,500-6,000. The off-season is cheaper, but still going to be expensive being a vacation resort.
And then there are the $250/month places in the middle of nowhere. :)