This is a subject I've been pondering and researching for a while as I'd like to be able to sell my photos for a lot of money. Wouldn't we all 
Here are some of my ideas and questions. Please excuse me if you think that I'm being cynical at times as you read the following.
1A) Image quality must be excellent but it's not necessary to be the world's best photographer.
1B)There are collectable photographers out there selling prints for a LOT of money that aren't nearly as good as others who are pricing for a lot less.
2A) If image quality is not paramount then what are the factors that contribute to collectability?
2B)Primarily it's marketing. Of course there are different marketing approaches to achieve the status that allows one to charge a lot of money.
3)The word status is intentional because it is one's perceived importance in the photography world that leads to high dollar sales. The word perceived is also intentional also because, even if a photographer's work is not terrribly significant artistically, if collectors and the wealthy think you are important then you're well on your way to the bank.
4) First question. Is it necessary to have a gallery of your own in order to have the appearance of importance in order to be taken seriously by collectors and the wealthy? All the photographers I have researched who are marketing themselves as collectable and are charging over $1000 for unframed 24x36 prints have galleries.
5) Second question: Is it necessary in your marketing to have the bragging rights associated with using a 4x5 film camera or bazillion megapixel medium format back?
This may sound truly cynical but all the collectable photogs do it.
1A) Print quality is extremely important to collectors who are spending big money.
1B) Some people are savvier business people than others. What defines 'collectable" are the curators and individual collectors. And there are different markets for different types of work. Someone who is interested in work by (for example) Richard Misrach, Andreas Gursky, Jeff Wall, Abelardo Morrell, Mary Ellen Mark or Cindy Sherman likely is not going to be interested in photographs made by people whose work follows in the footsteps of work by the old masters like Ansel Adams and Edward Weston.
2A) Image quality defined by whom?
2B) No it is not just about marketing, but being intelligent about who you market to and how you to is important. Also how is status awarded or won in any group?
3) There are multiple worlds interested in photography. For example There are a lot of photographers perceived to be important to other photographers (primarily for their mastery of technique). Likewise there are a handful of photographers whose work sells for tens and hundreds of thousands of dollars , marks, yen, & pounds who are looked down on by the first group because the first group doesn't understand what is important in that world. People who collect Gursky Morrell, Sherman, Wall, and Misrach aren't likely to be interested in Peter Lik. There might be some cross over for someone like Annie Leibovitz or Helmut Newton - but the interest there is mostly the subject manner.
4) No. Unless of course your market is tourists with money and the gallery is located in a high traffic location.
5) No. Remember what Arnold Newman used to say: "We make photographs with our hearts and our minds, Cameras are just extensions of our hands." What is important in the high art world is to make work that is considered important by the gate keepers in that world: curators and gallery owners. They see a stunning amount of photography, and derivitive work gets ignored immediately.
People who spend a lot of money on photography have a few criteria: If they are collecting just for themselves Do they like the work? Is it important to them? Some collect as a way of investing - they are looking for work they think will appreciate in value. Sometimes it is a matter of both.
You have to know the audience you want to appeal to.
