How much is an engagement ring? Mmmm
Well you need to work out how many years till you retire x your salary plus inflation plus all the debt you'll be in with mortgage bills and then kids who'll bless you drier than the wife
Yeh that's about it! I know I'm an old and you'll pay no attention to me and do what you want anyway.
Seriously though don't go overboard at the end of the day it's a symbol of your love and intentions save the extra money towards your wedding and house.