What is the typical cost to have a ball python for pet?

Let me say that in 25 years I have never had a ball python. Not my cup of tea I guess. But the principe is pretty much the same.

The heat pads are called under tank heaters, (UTH's). They electrical devices to stick to the bottom of an aquarium. They heat up. I personally don't like them, but they can be very good in certain situations. I am not sure what you use with fish, but I would assume they similar to the disposable heat packs we use to ship reptiles and keep the insulated box warm.

After the initial investment, the main costs will be, food, (a mouse or rat a week, after it reaches adult size every two to three weeks is fine.) Substrate change. And heating replacement, depending on the heating you are using. This won't happen constantly, but bulbs burn out, the UTH's die eventually. You may need to occasionally change out furnishings, branches, hides, water bowls, etc...

If you buy a baby ball, and put it in a 20 gal tank, (smaller is fine for a neonate, maybe even better,) at some point you are going to have to upgrade the whole set up. If money is a concern, as it normally is, you can set the baby up in a tub, (or as some of our British and down under friends would know it, a "click-clack"). A UTH can work great here if attached to a thermostat or rheostat. This is less expensive for the short term, then spend the money on an adult set up.