My girlfriend is pregnant and it may not be my baby. Please help me.?

My girlfriend is pregnant and she got pregnant 2 months ago. She has told me that she slept with other guys. I am really scared that it's not my baby. I love this women with all my heart. She is 15 years old and I am 16 years old. I want this to be my baby. How much does a DNA test cost? I will be there for her no matter what. I am not going to have her being alone with this abby. I will be a father for the baby. But I want to know if he/she is my real baby. Can anyone please help me with this.