What is a reasonable price to buy a diamond engagement ring for?

Im trying to figure out a reasonable price to buy a diamond ring that is 0.71 Cts. Round Brilliant cut, Polish-Good to Very Good, Symmetry-Very Good, SI1, Color Grade E. Comes with diamond certificate from E.G.L. also mounted on a platinum 6 prong. The asking price is $3,900. Is this a good price? if not what should I be paying?