I have lived in a studio for two months. The door seems quite thin because I can hear every move outside my room very clearly as if there is no door. How can I reduce the noise from outside my door in a way that induces least cost? I have filled every gap along the door but it does not help significantly. Thank you.
You've already done what you can by blocking gaps around the door.
Next, You can try hanging mover's mats from strong portable over-the-door hooks
Overlap the opening on either side of the door as best you can. Keep the length sufficient to clear the floor as you swing the door open and closed. The mass of the mats and their thick multi-layer construction will block most all but the sounds by contact with the floor from composition shoe heels and similar objects.
You can take them with you as they're not permanent. You can wrap the mover's mat to protect your belongings when you relocate.
If you buy new, when you get to a more private place, they'll make fantastic warm blankets on a cold day.
To Stan’s answer, I would simple add: You could also use an area carpet. Check Craig’sList —people give away these rugs all the time. You can find gently used ones and even new rugs for free.
A decorative carpet could be a more attractive and possibly free, sound-proofing solution. I found this area rug FREE on Craigslist just now.