Too right warpig. There are no rules other than:
a) the receiver must like the style, the colour (of the gem), the material (gold, white gold, platinum etc) the assumed quality and the manner of your proposal, and of course � you, enough to agree!
Money spent doesn�t really come in to it if all the above are ticked. If you can afford it and have 3 months salary saved up then why not splash out, providing you choose the right one of course, it could be thrown back at you! One set standard though amongst many women I know is that you try to, where possible, avoid Argos jewellery.
As you have said yourself, I would assume (although there is an endemic of materialism among many, but not all, women) that the recipient would prefer that you spent what was sensible to afford whilst leaving enough money in the bank for regular treats and/or future endeavours together (home, car, holidays, family etc).