If you are pregnant and don't want to spend much on a maternity belly button ring, check out the basic 5mm stainless steel ball and flexible PTFE Pregnancy Belly Button Ring. These are made from the most flexible PTFE available. They are very comfortable.
This one is only $2.99 and you can use the balls later on. After your pregnancy, you can use the 2 metal balls on the top of your regular belly button rings. They are 14g and fit on the end of regular belly button rings too. Since people tend to loose balls, these are great to have on hand in case you need them.
I also have several other flexible maternity belly button rings on sale in the clearance section. Most of these are under $5 each.