While I agree photos may be undervalued in comparison to cards to a degree, I think it's really not a fair comparison per se. Rare does not mean expensive. Just because there is only one copy of something, doesn't mean someone wants it or will want to pay a lot for it. This is true for every type of collecting that exists.
The reason cards are proportionally more expensive is because more people collect them, hence more demand. Even though there is more supply, the demand still outpaces it. As photos grow in popularity, you will see prices increase simply because the demand side will go up. I agree with those that pointed out that the registry is probably the single biggest thing that made graded cards the rage they are now. When I started collecting and dealing in 1978, people were perfectly happy arguing about whether a card was Ex, Ex-Mt, NrMt, or Mint. Beauty was often in the eye of the beholder.
Since grading became the standard, more non passionate collectors(ie: "Investors") got involved because cards became a commodity. As long as you had a slab, it was easy to buy-sell-trade with minimal effort or knowledge needed. All you needed to know is that you owned the highest grade available and could brag about it to friends, etc as a focus of prestige. Most of the people back in the 70-80s and earlier, the hobby was more about joy and passion, than showing off to people outside the hobby. Most of my friends and acquaintances had no knowledge or interest in the hobby, so showing off a cool t206 card with a rare back had no impact at all. Now, we have so many new to the hobby people with more money than knowledge. What is really sad, is that it does not seem many have any interest in gaining the knowledge as long as they can buy slab with a really high number on it.
No other part of the hobby can really match cards that way. Autos, photos, etc are all collectibles that really are not uniform. They require more research and knowledge in order to buy/sell. As Loren has expressed, this knowledge gap leads probably a fair amount of people to be hesitant about jumping in. Autos are a bit ahead of photos as slabbing for them started much earlier.
Photos, of all types will likely increase in value over time. Just like in cards, the more desirable it is, the faster and higher it will likely go.
My standard advice every time I am asked about collecting is the same. Buy what you like. Buy what gives you shivers up your arm when you hold it. Buy what makes you smile every time you look at it. If you buy with an eye towards investment/monetary potential, you will win some and lose some. If you buy what you love regardless of price, you will never lose.
I buy Gehrig items, because I enjoy them. I have a very eclectic collection of other stuff that is rare and much of it is worthless in the marketplace. Do I wish I had bought more cobbs, ruths, etc? Yes and no. I had a blast collecting what I did, so while there is a small part of me that wished i would have bought more ruth, Cobb, Wagner, Matty, etc cards, I am ok with the choices I made. I got to spend around 14yrs with my dad going to 35-40 card shows a year, hanging with a ton of cool, and some very unique, people, talking sports all the time, all while collecting some really cool stuff.
Every post needs a photo, so here are two of my favorites, both from 1927
