They dont have a cost in the contract (mine is a solar city contract as well, but from 2015 not 2013. I am sure its the same). it says something like they will have an appraisal done and charge "fair market value" for it. The issue is there is more than one way to determine value, and the way tesla determines value on it is based on the lost value of not getting the rest of the PPA agreement money / years.
I looked into buying mine out, they sent me the price, and the price they were charging for it was fairly astronomical. My system (for example) is 8.7 kW and they were charging me more for my 6 year old 8.7kW system than tesla would charge for a 12.X sized system brand new on my roof. When I questioned it, they sent me this 1 pager about accounting principles etc.
I asked another member here I know who does accounting stuff in their day job to look at it as a "non professional favor" to me, as I didnt understand what they were saying in the letter other than "here are reasons we feel justified to charge this much for buy out of your system.
End result was, it was due to the type of appraisal they used, etc, and while I could have probably continued to fight, I sort of gave up at that point. I may look at it again in a year or so.
Like most say, PPAs are not the best deal out there. When I got mine, I was making a choice between "no solar" and a PPA, and I have still saved money on it getting it when I did. I would be much better off If I had purchased the system vs getting a PPA, but it has still worked out for me financially (it didnt cost me money, I just am not saving nearly as much as I would be otherwise).
TL; DR -- I doubt the buy out price for the OP will be something they would want to pay, but it is not a contracted buy out price in the contract, its based on an appraisal.