This is a comparison of 0.5ct ideal cut, H color, VS2 clarity princess cut diamonds (click the dollar amount to be taken directly to the diamond):

James Allen: $990

Blue Nile: $1155

Abazias: $1216

These diamonds are independently inspected and graded by GIA/AGS, which means you can trust their quality. The quality of these diamonds is identical--the only thing that is different is the price.