Red line on screen after a drop (no cracks)

The key thing following a drop or water ingress is, if the phone continues to work albeit with a little screen damage, carry on using it but backup DAILY as it could die at any minute.

The cost if repair is the same dead or alive but it may not fail for years.

I recommend iCloud backup which will back up every night when locked, on charge and on wifi.

It could easily get worse. The screen could be starting to pull away from the body right now. But something definitely is broken.

If left unchecked by Apple Store you could find the screen stops responding and you have no way of making another backup.

Do not assume. You might use it for years, you might use it for only a week. I can only advise you to have it checked, and to find out how much money needed when/if it finally fails.

After having it looked at, you really should consider using a screen protector.