Dogs: Why is it so expensive to adopt?

Just now I was thinking how it might be nice to maybe adopt a dog. I'd have to wait a few years though until I had the room and responsibility, but I wanted to go on Petfinder.com to see the dogs listed from shelters ready for adoption. Well first I was astonished at how many dogs are in shelters, and it was just in Houston, Texas. But I couldn't help but be rather astonished at how much it cost to adopt! I understood something around $100 was understandable since the dogs have to be treated medically, and the adoption fees, etc. But some were charging $200-$400 to adopt just one dog! Some were pure-breeds, but still there were in a shelter! I would never have thought that it wouldn't cost $300 to adopt a dog from a shelter......ever! And I don't mean to sound rude or harsh in any way, but if shelters like those complain about how they have too many dogs then maybe they should consider lowering their adoption prices. It sounds like to me they are just trying to make money, and I thought shelters were more than that. I can honestly say after looking at some of the adoption fees they charge ( like $225 for a mixed breed rottweiler) I definitely feel a bit more hesitant about adopting dogs.

I'm not trying to bash on shelters for dogs or any other homeless animal. It's more that I just don't understand how so much can be charged for one dog at one shelter, when I see another get only a $100 adoption fee. Does someone understand this and can explain it to me, or does anyone agree with me that this is a bit odd and wrong?