On the whole topic of limited and tier pricing I have to agree with Brian and Bill.
I have been told outright several times by people that have collected my work and collectors in general when I was doing market research that they would not buy anything that was not limited period.
In a way I think limiting to smaller edition help me to get out there to make more image, which is a good thing as you are always pushing yourself to find more images.
I started a few years ago limiting to 60 and today I have dropped all my limited edition to 24 and I am thinking of even going smaller. It is marketing for sure but since this is what the collectors what I have no problem in doing it. When you rely on your prints sales for your living, you have to be smart about how you sell and how you market your material or it just sits in the drawer and collects dust.