Whatcha say

It doesn't mean "what did she say" but rather "what did you say." Right?

Whatcha say = what do you say/ what did you say. "Do" is more likely.
Whadya say = what did you say/ what do ya say. "Did" is more likely.

There's no hard rule though. When you're speaking sloppily like this rules aren't important. You understand by the context.

Whatcha say - what did you say or for eg. I will buy the car off you for 2000 pound, Whatcha say..?

It means "What do you say?"
It's a sloppy way of talking.