How do I repair a leaking shower?
No one can really tell you what the cost will be online. You're going to have to have a contractor come out and take a look. 

It could be as simple as replacing the tiles and grout.

It could be as complex as gutting the bathroom and completely replacing all the rotted framing (if it's rotted). 

So, could be $500. Could be $10,000.

There's no easy fix for this. Your best bet is to just top using it.