The engagement ring has a long history going back to caveman days. There are many meanings for the engagement ring. Regarding how much to spend is of course a personal decision specific to meaning while related to cause and reasoning.
The obvious is purchasing the engagement ring is it is traditional, however by purchasing one learns it is not of a tradition. The ring is fresh. It is not a family heirloom being passed down in lineage. That is where meaning related to cause steps in one may say IMHO.
I know couples who married and later purchased another engagement ring with greater value and too, seeking a sentiment. At times with the renewal of vows. I know of those those two times I have experience with a new engagement ring was purchased while with the renewed vows the original wedding rings were used again.
At question of course is means. How much does one have. Another is expediency. How fast does one wish to become married. At task could be again meaning, cause, and reasoning. Planning may step in offering a pause. Consideration may enter the picture with the relationship regarding financial planning.
Another element is the surprise as an element with the proposal. That with meaning most likely will have the most significance in many cases. The ring being a direct connection with commitment and the heart led by the soul may present itself with the formula.
Interest may present opportunity for a series of hubs on the engagement ring spanning history, tradition, valuation - financial, sentiment, and personal perspective. Again, purpose with meaning related to cause and reasoning offers choices too for a writing expedition. Possibly a lucrative adventure ventured.
Should a man be expected to spend 3 months of salary on an engagement ring? Maybe. One thing is the engagement ring is like a saving account sometimes. 3 Months or more may be reasonable to adjust to a change like separation, divorce, death, and etc.
How much would 'I' pay? I simply dun'no without discussing it with my significant other. :-)