There are many places in the $300 to $400 a month range (plus electricity) and even a few in the $200-300 range, though few "recreational" RVers patronize those, populated mostly by long-term residents in the workforce. $350 a month here for a site at a lakeside "resort" and less than $300 for a "workers" RV park a short commute into a metro area of 500,000 people.
There are also many places $1500-3000 a month, with special locations (like Southern California, the Keys) or special amenities (like marinas, 72 holes of championship golf courses). Wealthier people patronize those.
It is a matter of choosing the lifestyle that fits your budget. There are people who full time on Social Security or modest pensions, supplemented sometimes with work-camping or other jobs locally. Others might be budgeting several thousands a month. Except for the cost of moving around, it should not cost more to full-time RV than it does to maintain the same lifestyle renting sticks and bricks in the same location.
If you are seeing a really big difference in cost, you may be looking at RVing in places where the cost of real estate is much higher than at your current home. For someone coming from a place where a 800 sq ft apartment is more than $1000 a week, $400 doesn't look like all that much :)
The membership/condo option can work to lower costs, if you use the system consistently. You will find a wide range of prices there also, for the same reasons of location and amenities. Buying a RV lot somewhere can also save you money, if you are happy with staying in one place, or you can get a good return renting it out when you are not there.