Basement Pipe Leaking Water from the 2nd Story Bathroom?

Hi all! I don't post here on Reddit often, but I really need some advice from someone who knows what they're doing. My Mom's basement has some leaking water specifically from this big white pipe (behind the toilet), and some of the wall paneling surrounding it is mushy and moldy like wet cardboard. It's not a huge flow of water that drips out, but my brother uses the bathroom a lot and by morning there's a medium sized puddle around the area, which I just sweep down the drain in the middle of the basement.

What could be causing this, and how much would you guess it's going to cost? 😥