Find out if she wants diamonds on her wedding band or a simple wedding band of the same material as her engagement ring.
There are engagement bands with a "channel" setting, ones that have 3 fair sized diamonds, ones that have tons of small diamonds throughout the band, etc...
In my experience, there really is no "quota" for buying a wedding band. I would find out what she wants or go shopping together. If you can convince her that the plain band is best (use the "it won't detract/muddle the look of the diamonds on the engagement ring" argument) you should be fine expense wise (especially if the band is yellow/white gold).
As other posters have said, getting a matching set is common. Regardless, going shopping together is actually fun and exciting and takes a lot of pressure off of you to guess what she wants.