The price of an engagement ring is pretty straight forward coming form two parts: the diamond and the mounting. The diamond is of course the center piece of the ring with its dazzeling beauty and sparkle, while the mounting refers to the rest of the ring (the shank and the head).
Most of the price generally comes from the diamond. Since diamonds are highly valuable gem stones they can be very pricy. A major factor determining the price of the diamond is the desired quality. Quality is measured by the 4 C's which can be vauge and confusing. We have simplified this system into the A,B,C quality system. So, the A,B,C quality level chosen, combined with the size and shape of the diamond decide the price of the diamond.
The price of the mounting is determined almost entirely by the metal chosen. Platinum is the most expensive metal and it provides a high quality, white look. More commonly chosen options are 14 and 18k gold, be it white or yellow. So, the total price based on the costs of the metal for the mounting and the quality/size of the chosen diamond.