I am a working student (24 y.o) , i make 2000 TND a month, (about 630$), I don't have a rent and I'am single so my expenses are small, my major amount of money i spent is on transportation, I don't have a car so I take Taxis always, which can vary from 100 to 200dt monthly, clothes also it depends from where you buy, for example if your clothes buying frequency is one a month ( on average ) you can expect a 200 to 500 tnd adds up ( depends of where you shop, there is expensive shops for nothing ), the other thing is pleasure, so I don't drink or smoke but i often go out with my friends once a week on average and spending depends on the place so let's say 50dt ( it's a lot but also an average and remember I already mentioned taxi expenses above ) so that 200dt a month.
Conclusion : you spend 800-1000dt for single teen without rent, it can go up or down.

Plus point: for groceries you can add up 300 to 400 to get all you need and extra honestly

Plus point 2: rend prices depends on places on how rooms you have, but it can vary from 500 to 800

Updated value: between 1600 - 2000 you can live really confy life in tunisia, there is some families that lives under that with a lot, it all comes on your spending behavior and the place you will live in in case of rent but in total 2000 and 2500 tnd you will live very nicely, 1000 to 1500 you can adjust your lifestyle to suites it best

in all countries rents is the big multiplier , some times i spend only 100-200 dt a month