DES Option 2

Use the same rate in both models based on the rate calculated for the DES.

The gas price has nothing to do with it in this case since the DES does not use gas. Appendix G does not say that the Baseline is always gas. The Baseline uses the same fuel as the Proposed in Appendix G. See G2.4 - the energy cost is determined using the actual rate. The rates applied to each model must be identical. Section 2.4.2.2 of the DES v2 states that the rates must follow the normal Appendix G and LEED protocol. The DES guidance is set up to account for the impact of the DES. If the system is more efficient than the Baseline you show savings, if not you show a penalty. In general Appendix G and LEED do not allow savings related to the comparison of different fuels. The document is certainly not an official document so it does not really matter what it says but it is technically correct. If the DES used gas you use the building rate for the gas (not the DES gas rate) in both models. Since this does not apply to your case and 2.4.2.2 Exception a does apply, you use the central plant rate.

In the future please post your questions and comments in only one of the energy-related forums. I monitor them all and would prefer not to have to reply twice. There is also a DES specific forum - http://www.leeduser.com/topic/new-district-energy-guidance-leed-released-usgbc

CHP make it all a bit more complicated but the fundamental rules do not change. You need to use the same rates in the baseline and proposed. Since you are not using gas you apply the central plant rates. You cannot use a different rate between the two models.

It seems to me that you are taking one sentence and trying to justify a comparison that uses different rates. You need to look at all of the applicable documents, not just the one that appears to give you the answer you want. I simply do not have the time it would take to figure this out for you. It is entirely possible that there are contradictory statements within the official LEED documents.