Just like any other piece of jewelry, these rings―especially those made of platinum, gold, or those with gems and stones such as diamonds, rubies, and emeralds―can be very expensive. Not everyone can afford to give such costly rings. This, however, should not deter you from buying one as there are various inexpensive options available.
One of the best options is to go for the engraved rings. These are made in sterling silver and you can get a personalized message engraved on them. A love quote, your girlfriend's and your name, any special romantic line, or words that you have for one another are some of the engravings you can consider.
Some people engrave sayings in French such as "A vila mon coeur gardi li mo" which means "Here is my heart, guard it well", or "A Ma Vie De Coer Entier" which means "My whole heart for my whole life". However, if you do not want to write anything elaborate, simply writing "I love you" is the perfect way to convey your feelings.
Another option for beautiful rings are those which are made of sterling silver and are engraved with a number of tiny Cubic zirconia (Czs) stones. So, if your girlfriend prefers a flashy stone studded ring, this is a good option.
These days, sterling silver rings plated with platinum or 18k gold overlays are available under $50. Such rings are a good choice for those who have limited budgets but want the look to be similar to more expensive metals.
If you want to create a personalized ring for her, a good idea would be to buy a ring which has her birthstone on it. Australian crystal birthstones are a cheaper alternative and are easily available for those who cannot afford the real birthstones.
Some other inexpensive rings are the ones made from materials such as glass, wood, ceramic and bone. However, as it's a promise ring, preferably choose a metallic one, as they look more elegant and classy.