Thankfully, you can buy cheap diamond eternity rings by choosing the design and setting carefully. With reference to that, half eternity diamond rings will be way too cheaper when compared to the full eternity rings. In the former option, the gemstones will be paved up to the half only; that is, up to the visible area. Similarly, choose settings that involve less metal mounting such as a prong, bar, etc., rather than a channel or bezel.
Be attentive while choosing the metal to design your ring as well. Note that even though platinum is the best, it is better to go for gold so that you can cut the cost better. This will not affect much on the quality of your ring since gold is also a durable choice.