For an SI1 daimond, it is a bit on the high side IMO. Then again, you are paying a premium for all the analysis and documentation that comes with the stone.
It comes down to whether you are willing to pay for the assurance that the stone is a quality diamond (BTW the stone is very nice stone). If you would have been willing to paid for an independent assessment but for the documentation already included then by all means the premium would be worth it since the cost all washes out in the end.