Respiratory infections can be very easy to treat. Antibiotics should clear it up relitavely easily. Where did u get the snake from? How were the temps when you got the snake? What does ur set up look like! If he/she has had a round of antibiotics that should do the trick.most of these infections are caused by husbandry issues and providing the incorrect environment for them! I would recommend u invest in a good book or do a few internet searches for care sheets if this is his first snake! I always suggest to friends DO YOUR RESEARCH BEFORE getting the pet what ever it may be from a puppy to an beardie!
I took my ball python to a vet and it will definately cost you a bit to treat and sad to say if its serious the treatment mite out weight the money spent at the petshop substansially.
Antibiotics need to be used judiciously on all animals but if its not clearing up it may need a longer course. If its a young snake I do worry about using too long a set of antibiotics as well!