I first wrote about this market over a week ago. Back then, The Undertaker was 7/2. From there, his price began to slide and can now be found at 4/7.
In a post published yesterday, I pointed out that the Kambi oddsmakers have a similar market. They've adjusted their odds due to a report in this week's issue of the Wrestling Observer Newsletter that - as of the time of writing - John Cena .vs. The Undertaker is one of the matches planned to take place at the big event.