I would just show that you are responsible, that you can afford not only the set up, animal, and care, but you also have a fund set aside for vet bills. Show you are responsible around the house too. Take on more chores around the house. Do well in school and don't bother them about it. Asking them every day is not going to change their mind.

In the end, if they say no, they say no and it is best you not try and hide snakes in your bedroom.