Firstly, I must begin by saying that the 11th edition of the Britannica has attained legendary status for the quality of its articles and this means it is even more sought after and thus expensive that earlier editions.

I think that a complete 11th Edition in above average condition (some markings, maybe some aging of the paper, no obvious damage or tearing of pages, minimal creasing) should set you back at least 300 pounds, maybe even 500 pounds or more. That would be around $US600 or more. This is excluding shipping. Note: this is a rough estimate. Due to the reputation of the 11th edition, its value is hard to estimate.

You may be able to get a better deal by trying second hand and antique specialty booksellers in the UK.

You can also buy the entire 11th edition reproduced on CD-ROM (the link is below) for $US100, but to me at least, that is hardly the same thing!

I have also found a site with the complete 11th edition online for free. (linke below)