That's a tough question! I guess the answer depends on whether you are talking about a set when it is brand new, or a MISB older set, or a secondhand set.
Large set:
At the moment,
4865 is selling (secondhand) on eBay for as little as £60, which I think represents excellent value, provided it is a complete set (spare parts are not easy to find). This castle is full of features, like secret chambers and trap doors, plus 6 klickies, one of which (the
queen ) is one of the most stunning ones around.
Medium set:
When I opened
6891, Night walk aka 9156, I thought that was good value, with the UV torch in there. It's still available from Amazon.uk for £12.99.
The
worst value for money are things like Everdreamerz and the Naruto figures. I waited for a 30% off sale to buy some of the latter, but the prices were still eye-watering. Nice klickies though!