Like Chris1, I had to move out to my own place. However my daughter got a snake after repeatedly checking out all of the reptile books from the school library and doing her research. Saving pocket money and doing odd jobs to get extra can help, too. When my son saved enough for a Bredl, we sprang for the enclosure and fitout (which cost nearly as much as the snake!). Good luck!
Just tell them all the positive things about the snake let them do a bit of research and show them that your responsible.
Took me 5 years before my mum finally let me get a snake but when my dad found out he said "whats the point of keeping a snake?"