I would honestly get a plant 
But seriously, there is no such thing as a “cheap” or “easy” pet. There are pets that are easier to care for or cheaper than other pets, but all pets require time and money in order for them to live a happy and healthy life.
Take hamsters for an example. When people think of a hamster, they think of an “easy” and “cheap” pet. Well, they’re wrong. Let’s say you’re starting off with decent care. Everything adds up; you’ll probably end up spending at least $200 just to start off. Every pet has continued costs. 
If you’re not interested in getting a plant as I suggested above, there are animals I am willing to suggest for you provided that you have done enough research and you are prepared to be paying about $200 to start off and continuously buy bedding, hay and food. 
Gerbils are extremely fun pets; I know from experience. It is necessary for them to live in pairs/small groups however for a beginner I would recommend only having a pair (decreases the risk of declanning). I advise you to do some research on what to do if your gerbils declan; although it’s not seen as much with pairs as it is with groups, pairs of gerbils certainly can declan.
The minimum for 2 gerbils is a 20 gallon tank (with 20 gallons, a tank topper is very handy). I personally wouldn’t house a pair in anything less than a 40 gallon. Gerbils are very active animals and I believe that they require lots of space, similar to hamsters. Bedding depth is also important. I’d suggest putting at least 10 inches of substrate to allow them to make tunnels.
[edit] I forgot to mention that having a vet fund is necessary. My gerbil needed to see a vet and it cost me $150 just for him to get Baytril and for an exam. I’d aim to have around that much money saved up (at least!) [/edit]
These are just the basics; the gerbil forum has tons of useful information and it’s worth joining if you’re considering gerbils. 
Edited by ~musical912, 25 May 2021 - 08:43 PM.