The most desired diamonds are colorless ones rather than the colored stones. However, the former ones are really rare and hence expensive. It is better to choose diamonds with a tinge of yellow or brown that cannot be easily distinguished by the naked eyes, as these diamonds would be comparatively less costly.
Flawless diamonds are expensive. Hence, incorporate diamonds with small inclusions that will give a starburst effect or other beautiful upshots. Besides, use settings like bezel or flush, which will conceal small flaws for an affordable diamond ring.
Round brilliant cuts are one of the most popular diamond ring cut; they are less expensive because of their availability. On the contrary, Princess cut and Asscher cut diamonds are comparatively rare and hence expensive.
This is the main factor that decides the cost of a diamond. Do not go bluntly for big diamonds as it will be really expensive. If you desire a huge stone, choose multiple diamonds and set them with a halo or cluster setting in order to create an effect of a single huge diamond.