MARTA pricing might be the easiest of any transport system. It is $1.75 per ride. Are there "travelcards"? Yes, but to get them you have to go to a RideStore which can be somewhat complicated and out of the way. So if you are visiting, I would estimate your trips at $1.75 per trip.

Atlanta's Marta system now uses the "Breeze" fare collection system which is basically a ticket or a card instead of tokens. Check out the Breeze site for more details. It's easy to use and the card can be reloaded making it easier than carrying change.