Seriously though, both times i’ve been married, i proposed and then we went ring shopping afterwards. As has been said, they’ve got to wear it so it might as well be something they like.
I think the traditional looking diamond approach is pretty safe if you do go for it yourself. Both my current wife and my ex are the sort of girls you’d expect to get something modern/different, but both went for very old styled traditional looking rings to my (and their) surprise.
If you can get away with a few hundred quid its a result (we went the second hand vintage route), but if not then at least its some consolation that the wedding band is usually a lot cheaper.
If she does blow the budget, it at least gives you a bit of room for bargaining when it comes to buying a new bike 😀