We have a static on a site in UK. I could tell you what we pay / what the rules are, but it differs from area to area and site to site.
We bought ours for 9K. It's 10 years old, recently refurbished and tip top condition. 2 Bed (6 berth). It costs us 3K a year in fees and electric, gas is extra. Our site is only open 1 Apr - 31 Oct each year and has no extra facilities. There is no rule here for how old a caravan can be, just that it looks tidy etc. We love it and use it every school holiday and lots of weekends.
In the same area the biggest site has a 10 year rule. They have caravans for sale that start at 5K, but they only have 2 years left on site. Considering that some of the better ones can set you back 50 / 70 / 100K I think that is pretty unreasonable to tell you that you must buy another after only 10 years. That site charges 7K per year, plus electric, plus gas, but it does have on site shop / club / pub etc etc - I just dont think it is worth it in a place like this.
If you are thinking of buying in an area where there is a lot to do (where we are there is a the beach, a swimming pool, fair, amusements, parks, clubs with childrens entertainment, pubs with adult entertainment, cafe's, restaurants and fast food outlets and more) then I don't think you need to pay extra to have all that on your doorstep too - in fact we prefer being on a quiet site (lots of OAPs) because we don't get any drunks at midnight (except us!).
If the area you are thinking of is quiet, then you may want to pay the extra for entertainment for children. DD loves the beach and walking up the Fort with us - but would still rather go to the fair!