All turtles need to be given at least 50 to 100 gallons of water, as well as a good amount of land, even water turtles need land with a heat area with heat and a UV bulb. Even the samllest turtles can get to be at the very least 7 to 10 inches. They also can easily live for over 70 or more years. And you need to do a full water cahnge every week, at least once a week.
Having a turtle (and taking care of it well as you should) is going to be very expensive, figure on at least $300-$400 for the set up to start with.
Tank (at least a 50 gallon)
Heat Lamp
UV Lamp (both for day and one for night)
Large gravel (needs to be big enough so it can't be eatten)
Constant fresh veggies and pellets (never live food as if can make the turtle sick)
Stand for the tank
Water cleaner
Basking pad
High power water Filter
Also keep in mind that turtles are an animal that does not like to be played with, you wont ever want to take them out much.
And if you don't keep the tank clean, not only will the animal die, but you can get very sick.
If you don't care if it's unhappy and dies in a few years then it can be cheap.
The people on here who say it's easy don't know what they're talking about, and clearly there's a reason that their turtles have died. Also never buy a turtle under 4 inches, this is not only wrong as it's to small to move away from the original breeding pond or tank, but also it is illegal in most states.