This can even be an anniversary gift. Imagine the look on her face when she realizes she will not have to keep wearing a diamond-less band and embarrassing herself. Not only does this make a great deal of sense, it is also a popular option among couples.
You can add diamonds to the band at intervals, getting them set flush into the metal. Otherwise, you can get the channel set or even grain set. Of course, the cost of the plain diamond ring will go up with the number of diamonds.
You already have a band, so you will not have to spend on the ring metal. Only the diamonds and the labor it took to make them will cost you. Moreover, note that what you pay will be based on the size and number of the stones. The shape options for the diamond will be limited by how thick your ring is.