Issue 150 in January was the #1 selling comic that month, even over Marvels. The problem is the comic gained tons of steam from the show. So a good user population was a show fan first, comic fan second. So those people would be pretty upset if the show/comic didn't stick close.
Issues #1 was 7500 comics and 2&3 roughly same.Issue #7 broke 10,000 copies.Issue #36 broke top 100 chart. Issue 75 broke 30,000 copies coming a year after the show was announced but prior to the first episode.
Now it is triple+ those numbers from before the show. So the show drives the comic sales not the comics drive the show.
Show came out on comic #78 and comics 100ish we saw the last 2 weeks was comics in mid 2012 or basically season 3 in the show.
If you want more information on that
THE WALKING DEAD - complete print run history - InvestComics