in my opinion (after 6 1/2 years doing rescue & adoptions and working with a Herp Society), a ball python isn't the best "first" snake, although they are typically docile. they are so docile people are often bored within a few years and move on to more exciting or interesting species. if you like the small size, I would encourage you to also look into Antaresia python species, and even Liasis fuscus.
I absolutely would NOT recommend a UTH unless you are going to buy a dedicated thermostat. my ball python has been living happily in a 40-gallon breeder with only a CHE (30W in the summer months, 75W in the winter months). I keep him in the herp room, where ambient is about 75-80 during the day and 65-70 at night.
as an adult, he eats 3-4 times a month. I give him medium adult mice at each feeding each mouse is less than $1.
I would say the total maintenance cost averages ~$15-20/month. he has a naturalistic soil-based substrate topped with cypress mulch and oregon tree moss. I spot-clean weekly and swap out the moss/mulch upper layer about every other month, and re-do the entire tank twice a year. aspen would be slightly less effort although with similar expense (and in my opinion is rather boring).
the main problem...what will you do if/when you go to college? dorms rarely allow them. will your mom keep the snake if you move out of state? if not, what will you do with it? like any other rescue, herp rescues are flooded.
good luck with it. snakes are fun to keep. I have several species of smaller pythons and I love them.
one more thing: if you have a nearby rescue, either a shelter or herp society, you might be able to foster-to-test-drive and spend a little time living with an animal before you commit. some places allow this and it's a good way to be sure you have a good fit before you commit to 30 years or more of ownership.
