Not as much as they used to be. Vol 91 used to go for £100 or more, but over the years, Ebay has grown and it's no longer such a rare thing to find. I would say £20-£30 is realistic, but it will fluctuate.
Others to watch out for are volume 92, the Best of 1984 collection or the other 'best of year' albums if they have the calendar posters inside.
Cassette editions of any albums from the series are also sought after, but there are fewer collectors of these.