Allotment sizes vary enormously, as do charges and almost every other facet of allotments. At our site, there is a system I haven't heard of anywhere else, in that the price applying when you get your plot is set for as long as you have it. People who have had theirs longer than us are paying £12 or £15 per year. We pay £20. People who took theirs on after us are paying various amounts up to £50. Many sites are council property, but ours is property of a local landowner and has been allotments since 1917. The site is administered (very loosely) by a local estate agent (realtor) who also handle the other business of the landowner.
The rules about what you can and cannot do or grow vary from site to site. We for instance are not allowed to keep any livestock on the plots, but some places do allow chickens etc. Our site has no water provided, so we catch what we can from the roofs of the shed and greenhouse. As to value, I believe it is enormously worthwhile. We buy nothing but seeds, seed potatoes etc for ours, various plants have come from friends etc. when a small punnet of soft fruit can cost £3.99 at the supermarket, and we can pick a washing up bowl full of blackberries, you bet your life it's worth it. We also grow rhubarb, raspberries, strawberries, blackberries, tomatoes,golden raspberries, gooseberries, and we have an apple tree, a plum tree, and a cherry tree. And that's before any veg.
But for many people the true value of an allotment is in the side benefits, ie fresh air, exercise, free or at least cheap food, improved diet, new friends, the knowledge that what you're eating has been grown by yourselves, and grown organically/ethically etc according to ones own convictions. And for many of us, the chance to make things out of junk and scrounge things out of skips etc.
Steve