Invincible tends to swing from a single issue perspective. Right now, #1 is selling around $50, but it has done a steady $150 in the past.
Really, it's the book that should be more accepted by the comic community compared to TWD. Invincible is a super hero book. However, most comic readers still keep their Big 2 blinders on in regards to super hero books, if not all comics. As they start getting older and their tastes mature, more and more readers will be attracted to Invincible (in my opinion).
Keep your eyes on ebay. Slabbed 9.8s come up every once in a while. At one point, they moved for $225-300. I recently picked one up for $60 shipped and I've seen them go as low as $40 shipped. I have raw 9.8s, but at that price, why not?
I agree with issues up to 13 with 1, 11, and 12 being the toughest to find.
Some of the second print issues, I never come across.
Also issue 50, 60 and 63 seem to be tough to find. I've been able to flip copies for about $10 pretty consistently.
Invincible remains my favorite comic. I get a bit OCD when it comes to runs of this book.