You can buy them individually, but they are $15 each ($17 if you want 8X10). DD loves characters, so we usually wind up with a couple hundred photopass photos. At the $149 preorder price, memory maker is the cost of about 9 photos, and as many as we get taken between the characters and the park icons and other places, I know there are going to be far more than that which I am going to want. It is worth it for us.
Also, if you are doing one of the character meals that has a photo other than CRT and Akershus where they are included, I think those photos usually run about $30. With Memory Maker they are included. Assuming Donald still poses for the Tusker House photo, that is one we pretty much always want. (Some of the others are against really boring backgrounds and we wouldn't spend our money on them if it wasn't for having them included already).
I think the ride photos are around $15 too. We've have a bunch of those that we wouldn't waste the money on, but every now and then we get a classic that I couldn't leave behind.