WHY ARE SECRET 7 BOOKS MORE EXPENSIVE THAN FAMOUS 5?

[BridgetJonesPants]

DD is 7.1 and at the moment, I think she would find Secret 7 easier to read than Famous 5. However, have been checking on line and notice that the cheapest Secret 7 collection (10 books) I can find is £23.82 compared to £9.99 for Famous 5 (also 10 books).

I've been keeping an eye on prices for the past couple of months, but the Secret 7 have never changed much. I accept the £9.99 price is a special offer that probably won't be on for long and next nearest is £16.48, however I still can't understand why there's such a big difference in price.

Any thoughts or anyone know where I can buy Secret 7 books cheaper. (And, before any one says, I know that £2.38 per book isn't going to break the bank, but I do like to pick up a bargain).

PS, meant to say that I've already looked for books on e-bay & gumtree.

Less popular I'd expect. You probably don't need the lot of them too. Mine only read one or two before finding Famous Five was much more fun and moving onto them.

WHSmith had the whole Secret Seven (15 books) on offer for £13 yesterday. I noticed as I'd recently sold the lot on Ebay for £12.
SS have been reprinted fewer times.

Cos there are two more of them?