Hmm. Let’s get slightly controversial. Browsing for maths dictionaries I found ‘A Maths Dictionary for Kids’ by Jenny Eather on CD on the Cambridge-Hitachi site. Now it was difficult to find a price on this site so I went to R-E-M and also found it there, for sale for £115.09 inc VAT.
Now I know that this is an excellent dictionary as I have used it before – great graphics and clear explanations – but over £100 seems rather expensive. And it seems even more expensive when I can access it free of charge at A Maths Dictionary for Kids by Jenny Eather.