You forget many things that entail snake keeping... It's not just sticking them in a glass tank and giving them a little food.
Herpstat are essentially are a requirement if you want to play it safe with high-end animals. Herpstat4 for 3 animals is $340. If you go with terrarium, better to go with an RHP, which are $40~$60 each with one per cage, so about $150. If you want to play it safe with ball pythons, you really need a pvc cage to use for your enclosure rather than a glass terrarium if you're not using a tub setup, which glass terrariums do not hold humidity (unless you want to cover the top with foil... but it doesn't look too good). $200~$250 per cage per animal, so that's $700 if you have three animals.
If you get adult ball pythons, you will not get 50 small rats for less than $50 lol... Fifty small rats would be about $100 including shipping and that will only last you 4 months. So it is actually about $250~$300 a year.
Ball pythons won't lay eggs annually. Females will often skip a year every 2-3 years.
Of course if you are counting in the long run, for sure you'll earn the funds back and more if the prices of your morphs didn't depreciate by a lot. But you won't be "making extra cash on the side" at best you break even in 2 years.