I've not been married nor have I proposed nor have I given either much thought, however, I've heard a couple rules that I plan to follow if/when I do find the "one".
1) Spend 2-3 months gross pay on the ring.
2) THAT you're popping the question should not be a surprise, WHEN you're popping it should be the surprise. Take her ring shopping. She should know that you plan to ask her to marry you, but the mystery should be in the timing and the ring. She knows your financial situation, and she can probably more effectively guide you to the price range she expects given your situation.
You're going to spend the rest of your lives making financial decisions together, why should the ring purchase be any different? Think of it as training for your future life.
EDIT: Don't listen to what she says when you're ring shopping. Watch her eyes, see where they go in the cases, see what settings or cuts or colors make her eyes light up.