As has been said, they an end up costing you a fortune. For several of the reasons already given, I wouldn't think of it as 'buying' - more like having it on a lease for up to 10 years, but one which can't be renewed or which will have any value at the end of the decade.
Depending on the size of your family and how much space you'd need, if it's just a bolt-hole you're after, might it be worth considering getting a used touring caravan, but finding a caravan site that lets you leave it there all year, or at least for the extended season (often April to October)? Even if a site isn't open for you to stay over the Winter, they still may well let you pay a small fee to leave it there permanently.
We go to a site once or twice a year with our tourer, and they offer seasonal pitches. We asked for the prices and they were extremely reasonable, especially the price for the months outside of high season. If we lived nearer and didn't want to go anywhere else with our caravan, we'd definitely consider it.
Going on the one where we asked (in north Norfolk), you'd probably be paying £1500-£2000 for the year, all in (apart from your gas). Obviously, you have to supply your own van, but there are no restrictive rules about how old it can be before you have to replace it or being forced to sell it to the site owners for a very low price.
Also, I'm sure that there are plenty of decent, honourable people who run static caravan sites, but it's renowned for being a business that attracts a lot of greedy, controlling and unreasonable sorts. They're often very demanding and will be constantly on the lookout for new ways of making more money from you e.g. forcing everybody to put up a new/replacement fence/veranda around their vans, to their specifications, using their own specified fitter (likely kickbacks involved) and paying an extortionate amount for it. Not all of them, as I said, but it is rife.