I had a look at your capx and without re-downloading it I remember that "Total.Text" is actually the string "SAVEDCASH" that you typed in the textbox properties. Using what Jayjay has suggested "Total.text" has to be a number and since you never actually update "Total.Text" in anyway it will always be "SAVEDCASH".
Here's a method of what should work, not at my computer with Construct so can't double check.
Start of layout 1 (where you tested for if webstorage existed)
Set "Total.Text" to whatever is stored in webstorage with the key "Total" if there is no "Total" key set "Total.Text" to 0.
Then in the layout where you added the score when you end that layout (can't remember how you did that) use Jayjays method.
WebStorage set local key "Total" to "int(Total.Text) + int(Cash.Text)"
Personally I would use a global/instance variables for any numbers that need to be added together rather that what is stored in a textbox but that's just me. Also I would highly recommend updating Construct to at least the new Stable, you said you were using an older version, updating would be much easier for people to help you in the future by providing small examples. I fixed the capx the other day but deleted it when you said that an example would be no good