Do it once but not twice. $40/per person for visiting the world's 6th tallest free standing structure is probably worth it.
If you're a cheapo, then you're never going to know what it feels like to be in the world's 6th tallest free standing structure.
I suggest you take a long look out in all directions and take a minute to think about your life, earth, work, family, and death.
Yes, the glass floor isn't big. But it is kind of scary if you're afraid of heights. If you're not, then by all means, go sign up for the edge walk. I dare you.
There are drink vending machines on the floor for your pleasure.