Should the value of the legendary item count against Wealth by Level? I tried searching but I was unable to find anything.
I'm leaning toward "no" because you only have precious few mythic abilities to choose from so you are still investing. However, I can see an argument for it since you are getting a lot of value for no monetary cost.
Yes, the value of a legendary item count against WBL.
However, legendary item properties doesn't have a cost, so only the "base" item cost would be added.
The Upgradeable property is particularly illuminating in this regard. Presumably, both that ability and the appropriate crafting feat would suffice to reduce cost for upgrades... thereby indicating an established monetary investment that additional magic would require.
As WBL represents the average retained monetary investment over the course of a career, it is easy to conclude that regular magic on the legendary item counts against WBL. (As others have specified, mythic abilities would not increase the cost)