Back to the OP, I'm afraid the worth really depends on the person leading it (and the person being led, in terms of what they like). I have been on very worthwhile guided walks, usually by people who can give me information I didn't already know or show me things I couldn't find for myself.
The price would depend on the experience/qualification of the person running it, the terrain it is in, how many people are in the group, and what exactly was involved.
Unless they're charging just to cover expenses, or they are subsidised somehow, they will probably be looking for about £100-120 a day. The more people in the group, the more that can be split between.
Some examples that I've led (all requiring them to bring their own food and clothes, but with me providing any technical gear):
custom day for two people doing some Lakeland ridges - £200
day looking at Bronze Age settlement and ritual on Dartmoor for up to 6 people - £20 a head