AppleScript sum of a list
Modifying your original attempt:

set myList to {"30.00", "30.00"}
set myFinalTotal to 0

repeat with x in myList
    set myFinalTotal to myFinalTotal + x
end repeat

myFinalTotal


And for the joy of learning:


Recursive:

to sumItems from L as list
    if L = {} then return 0
    (L's first item) + (sumItems from the rest of L)
end sumItems

Iterative:

to sumItems from L as list
    tell (a reference to last item of {0})
        repeat while L ≠ {}
            set the contents to it + (L's first item)
            set L to the rest of L
        end repeat

        return the contents
    end tell
 end sumItems