There are many things you have to keep in mind. An animal is a money and time pit. You could easily say that the cost for food, training location, and other time where the player could have been making money he was instead training and taking care of his dog would easily make up the difference in cost. But the starting money isn't necessarily like he suddenly decides to go adventuring and finds himself with X cash that he needs to spend. Instead it represents years of training, odd jobs, and side work that has resulted in the accumulation of these items which he now has. By spending time on the riding dog, he didn't spend time acquiring other objects of value.
