Anyone who thinks #100 is going to increase in value is dreaming. They are "collectors items" Any time something like that comes out...it's destined to not be worth much, if at all.
Walking Dead 1 is worth what it is because it was a low print run, sleeper comic that wasn't ever thought of as a sensation. Hell, they were lucky to get to #5.
Some issues like 19, 27, 33....they are what anyone would consider keys of the run and will garner a premium to others in the run...and anything 100+ is most likely not going to be worth anything. The print run has to be massive. Especially if I am subscribing....I haven't bought new comics monthly since the 90s. So you know the print run must be massive.
I did buy a Kirkman/Adlard sig #100 McFarlane cover 9.8. That cover is really cool. The SS makes it worth it for me.
The cool part about me buying WD comics is that i'm really enjoying the writing and that's something I haven't done in years with moderns.