Welcome to the forum
The price depends on many things, location, amenities and size, for example. London allotments are always going to be more expensive than equivalents elsewhere.
I have a 5 perch plot in Asford, Kent with no amenities, except the site's perimeter fence and water taps only turned on for the summer months, and this costs £36.
Pensioners and others on certain benefits get a 50% discount.