This is wrong angle of looking. You are talking about engagement ring, not just a ring. The word engagement is first because there is reason behind it. You should not focus on just ring, you should focus on fiancée. The engagement ring is even not required necessarily.

If you can afford its okay, get one. If its 3 carat rock with sterling silver band its okay. Some jewelers may ask you for 10% of annual income or your 3 months salary. But according to etiquette experts these are outdated horribly and it is just for people to spend more on engagement ring design.

There are many sites you can design engagement ring. If you are looking for diamond center-stone engagement ring design I personally recommend you BlueNile.com. They have outstanding customer service. They offer unique and matching wedding bands. Only choose those rings which have three or more stones, so you can put your thoughts into the purchase. You can see at your local jewelers if your fiancée prefers gemstone jewelries. I have also seen many elegent sapphire and ruby engagement ring designs out there.