Price can vary widely when Carat size, Color, and Clarity are taken into consideration.
Carat is a measurement of both the weight and the size of a diamond and will determine pricing followed by the additional C’s. 
Cut Diamonds are being cut to different shapes; the traditional round brilliant,classic emerald cut or even ovals they all have their unique light performance and their differences in pricing. 
Color White Diamonds are graded by letters D (being the whitest and most colorless) and all the way down to Z (which appears to be faint yellow). When designing with white gold D-I colors are a safe range while yellow or rose gold can allow more tolerance for lower colors J-L
Clarity  Determines how easy it is to detect the imperfections inside the diamond using a scope and how easy it is to spot them to the naked eye is the diamond eye clean face up is a major factor.Flawless being the clearest and SI2 (slightly included ) will cary a fewer more noticeable imperfections VS1 - SI1 is for sure the safe zone.
read more gemological facts here GIA