3-4k for me. 2 months salary was reasonable 30 years ago, but now it really depends on what she wants (also not what she tells you she wants, but what she really wants).
Check out bluenile.ca. You can select your diamond, see the grading resorts and choose your setting. You'll get more diamond per dollar than at most jewelers. Make sure you know her absolute size as there's no resizing on Canadian orders.
Some good threads I read here about the same topic:
http://forum.calgarypuck.com/showthread.php?t=74543
http://forum.calgarypuck.com/showthread.php?t=71248