This doesn't appear difficult. Steps 1 and 3 can be gleaned from examples at the iworkautomation.com site. The details can be worked out if this approach looks as if it will be efficient for you.
Step 2 is less obvious.I created a three-slide document. Slide 2 looks like this:
It turns out that Keynote treats this as two 'text item' objects on slide 2 (the title at the top is text item 1, the bullet list text item 2. So we need to make the replacement in text item 2's property called 'object text' (I got the name of the property from the Keynote dictionary accessed via Script Editor, in Applications > Utilites).
So, in AppleScript's typically somewhat verbose fashion, the find-replace is easily made with a script like the one below.
To run the script, copy-paste into Script Editor, make sure the target document is frontmost, and click the triangle 'run' button.
If you're placing a value in a cell in a table on a slide (as opposed to making a replacement in text) then the code would be more succinct.
Anyway, it does appear to be quite straightforward.
SG
tell application "Keynote"
tell front document
tell slide 2's text item 2
set tt to its object text
set tt to my replaceTxt(tt, "$Total", "$100")
set its object text to tt
end tell
end tell
end tell
to replaceTxt(tt, f, r)
set AppleScript'stext item delimiters to f
set tt to tt's text items
set AppleScript'stext item delimiters to r
set tt to tt's text items as text
set AppleScript'stext item delimiters to ""
return tt
end replaceTxt
