Don't know what "bottom" might be for WD comics, but rest assured it will go down, significantly. If we're already seeing a drop in prices, and I think most agree we are, and the show is still red hot, what's going to happen when there is no show? Merchandising is limited for the property, and zombies won't be the "in" thing forever. Might be fading now.
Remember, book is only at the level it is because of the show. That drove many to dip their toe into the comic waters to pick up back issues and current issues. For people that haven't been into comics before, they're one and done. Not like they're crossing over to other books.
My guess is, couple years after show is off air, the lack of interest in the property will cause a sell-off. Less demand, higher supply, Econ 101 tells us what will happen to values.
IMO, of course, but pretty confident.