I've had some semesters that are higher, but normally around $400-500 without supplements. I normally try to look for books that have no markings in them so I end up getting a mixture of new and used books.
You could call the bookstore that usually services your school and ask them about an average price for 1st semester 1L books (get different quotes for all new and all used). The prices may change from semester to semester because the books used change, but they can give you an idea of a general total for the current semester.
You may want to also get a list of books being used for the current semester and then check on-line for retailers that sell law books. That will give you some idea of an average price.