This is such a personal question, and nobody can really answer this for you. While people in the jewelry business favor advising clients to spend the equivalent of two months salary, this will only work if you have two months salary saved up. While of course, an engagement ring is an investment of sorts, it won't be something you will ever sell, so it isn't as if you will get a return on your investment. As you begin looking for rings, ask yourself some of the following questions.
1. What you can really afford?
2. Do you want to set up payments?
3. If so how long will it take to pay the ring off?
4. What will the rate of interest be?
5. What other bills will you and your fiance have as you start your life together?
6. Are you going to make another large purchase soon, such as a vehicle or a house?
7. Do you have job security?
8. Are you expecting a raise soon?
9. How important is an expensive ring to you or your fiance?
Once you've answered these questions, you should have a better idea of just how much you can and should spend on a ring.
From
http://engagementrings.lovetoknow.com/wiki/Budgeting_for_an_Engagement_Ring