Can't put a number on it for a specific day, but to give you some perspective, typical W to J upgrades on ex-US routes range from 499 USD at the cheap end to 999 USD at the steeper end. There may be prices outside of this, but this would give you a typical range. Flying out of LHR, 400 GBP to 850 GBP a typical range, but again may creep outside.