"We" spend on average three months worth of salary. Around £5k I guess. Certainly not enough for a house, but a good amount of money to put towards your first family home when you get married. Houses need furniture and all that fancy stuff. :)
I'm British and I've never heard this three month thing outside of the US. Surely this isn't a UK cultural thing too?
Never been married either, mind, or in a position to propose, so it hasn't exactly come up.
A mortgage is usually on three times annual salary, so 312x = 36x. A ring is 'worth' 3x, so not far off a tenth.
The silly/sad/stupid thing is when you put it in context of a ring+marriage ceremony. The same way many college students are starting out life with debt, they often go and add 25-30K of extra debt between an expensive, value-less ring and a wedding ceremony.