First you want to start by thinking if you have the time and money for reptiles and amphibians. you need to know they live up to 30 years.
Most of the cost will bit up front most costly things being:
the snake (or other reptile/amphibian)
On a Bi-monthly basis you will need to buy:
Food (aka Mice)
Bedding (depending on size of bag/size of cage you may only need to buy bedding 1-4 times a year)
There are a few different ways to get a snake: Pet store; Animal shelter; snake breeder.
There are other ways to acquire a snake. I got mine free from a local volunteering group, i have been helping for years and when one of the snakes we use to educate people laid eggs i got one (adding 15 babies snakes to a family of 10 is just insanity so most of the volunteers got one for free)
I strongly recommend getting a Corn snake
or Ball python
if your beginning. Both are easy to care for, common, docile, and non aggressive (and non-venomous). Ball pythons grow up to 4-6 feet long and get VARY heavy so a Corn would be better for smaller children. Just to clarify these are not the snakes that you hear in the news, starting small and growing to 20 feet, eating dogs cats and people.
Ball pythons will need larger food than snakes ( Rats v.s Mice). They are native to the Pakistani area but it is vary common to see baby snakes in pet stores for $80-150.
Corn snakes grow 3-5 feet long are faster than ball pythons eat slightly smaller food (mice V.s rats) need less space and are native to US. They are not as
common in pet stores but should be easy to find from a breeder $50-150.