Well, I might have rounded the numbers to the orders of magnitude :-) but if one likes 'seasoned' books (to me 'seasoned' means 'not dumbed down'), you can save a lot of money.
For example, I bought this
http://www.amazon.com/gp/product/0137127952/?tag=pfamazon01-20 for 9 pounds.
And I had Jenkins White "Fundamentals of Optics" for less than 5 euros, shipping included. If you look up this paperback edition
http://www.amazon.com/dp/0072561912/?tag=pfamazon01-20 it costs 160+ pounds. (Ok, I am cheating, I bought this one:
http://www.amazon.com/dp/0070853460/?tag=pfamazon01-20 ) but it's the same book.
Sometimes I can have a book for a ridiculous price just by getting one or two editions before the last one (this could also mean less useless colour). Check out Devore's
http://www.amazon.com/dp/0538733527/?tag=pfamazon01-20 - the eight edition is 136 pounds (hardcover). The sixth edition hardcover I had for 5 pounds.
Of course really good books do not come cheap, because people tend to treasure them. :-)
But you can find real bargains, if you look well.