There are a few articles floating around the web on how the human mind naturally preceives numbers on a logarithmic scale. Scalar abstraction of numbers is only a function of education and conditioning.

What this essentially means is that given a range of numbers, say 0 - 100, we see 1 and 2 to be further apart than 99 and 100. An article (somewhat heavy!) about the subject can be found here: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2610411

This may go some way to explaining why we don't particularly mind paying an extra €50 on the price of a €7000 car but wouldn't dream of paying €51 for a €1 coffee. The end result is the same, we lose €50. It also may explain why we tip more in a restaurant to a waitress that's done the same amount of work as a waitress in a café or bar. (Thanks to Al Jafee of Mad Magazine for the latter observation made sometime in the 70's).

Now I know why I failed mathematics at university!