I am flying to SFO Premium economy for my holiday and i am wondering ....If there was availability on the day to upgrade to ucs what should i expect to pay.
Depends entirely on your ticket type. What class do you have on your PE ticket? W/S/K/U?
That, of course, excludes "airport specials" which could be around £500 - but be warned, there are a lot of Golds on the 19/20 route - all with miles to burn, so the seats go quickly...