In "The Price of Gold", if you look closely, or play it in slow motion, as Cinderella signs Rumplestiltskin's contract, the first letter she writes is a C; seemingly indicating that her true name is Cinderella, and "Ella" is simply her nickname.
However, the Season Six episode "The Other Shoe" canonically establishes that her real name is Ella, while her nickname is "Cinderella".