I spent 3.5k on one last year. Just sold it for £900, it had acquired bad damp in that 12 months. Cost £200 to have a tow bar fitted.
Spent a few £100 on various bits, locks, sleeping bags, gas, steps, cutlery, aquarolls, awning, etc.
Just spent 10.5k on another caravan. Mhopefully a not damp one this time!
It costs me £200 a year to store it. You're looking at about £150 for an annual service but you could get away with alternate years in an older van as long as you have a carbon monoxide alarm I reckon.
£46 a year for caravan club membership.
Site fees vary. I'm spending £12 a night on a weekend away next month on a little site with electric hook up and not much else. Other sites in the summer can easily be £25 a night, and these are sites which don't have clubhouses, entertainment, etc as I don't like those.