In normal circumstances, I would assume it wouldn't raise as much value as it cost, however...
We live on the edge of a row of houses with the pavement running the length of the garden. The pavement is approx 3 - 4 foot higher than the garden, and we currently only have a small metal council fence (about 3 foot from the pavement) separating the two (and a wall our side up to pavement level...) We have a shop at the bottom of our garden and a pub across the road from our garden so this pavement is used a lot. Also there is a bus stop there that will become a bus shelter come February. 
We often get old biddies walking their dog and stopping for a good nose into our garden (and kitchen..), teenagers chucking rubbish in, and the bus drivers throwing their fag butts in.
We are hoping to move in the next few months. It's a really gorgeous house and we're trying to make it seem like a real family home despite the fact we live by a pub.. So.. do we add the fence?
Also, should add, that the land next to the house (kinda inbetween the 'official' pavement and our garden, but used as a pavement) is for sale, (which we will really be pushing to prospective buyers, as it will mean there is more than enough room for an extension... and/or a small detached house at the bottom of the garden) and so if the buyer bought that as well, the fence we added would be a waste.
The cheapest we can do the fence for (if we do it ourselves) is £1300, or around £2000 to get it done properly. I have checked into planning and we don't need planning since the land in between is privately owned (and that owner couldn't care less as he didn't know he owned the land until we told him...) Also the council fence would have to stay, but we would attach the fence to that and either hide it with bamboo screening or encourage some climbers onto it (though not in time for sale...)
This turned out to be a very long winded post.. thank you if you have got this far! :)