It’s definitely possible to go into seminary right out of high school. One of my dearest friends went to seminary right out of high school, and he is loving it. As for the costs, that would depend heavily on your diocese. In my home diocese, a seminarian’s tuition is free so long as he is ordained; if he leaves, he has to pay.
In my opinion, though, you should very seriously discern whether or not this is the right decision for you. A few priests I know went to seminary right out of high school, but left. They returned later, but most of them feel that they were not meant to be in seminary right away. God has different plans for different folks. Find out what He has in mind for you (as I’m sure you are.)