You can get them on ebay for 99p. It's only the older one's that are worth any money. Even then they have to be in excellent condition with dust jacket included.
The value of an old book depends on 2 things - 1) its rarity and 2) the number of people who want it. In this case, I'd say, not rare enough , and not enough people wanting it to push the price up.
Unless it has Princess Diana's signature in it, or something else which makes it special.
I bought an entire set of Chambers encyclopedias from 1905 for £20 in a book shop, 8 books, mint condition. Because they are not rare, and they are not particularly desirable. I only wanted them to make my shelf look good!
I agree with others here, not a lot, but very much worth hanging onto for nostalgic reasons IMO. The older ones were far less glossy and far more full of interesting and varied stuff. Shove it away somewhere then dig it out in about 20 years time and have a good read through !