This script will move a page item regardless of the dimensions of the artboard.
global artL, artT
tell application "Adobe Illustrator"
set artRect to artboard rectangle of artboard 1 of document 1
-- left, top, right, bottom
set artL to (first item of artRect)
set artT to (second item of artRect)
set myObject to first item of selection
set myObject's position to my convertPos(myObject, {0, 0})
end tell
on convertPos(pageItem, itemPosition)
tell application "Adobe Illustrator"
set cWidth to (pageItem's width) / 2
set cHeight to (pageItem's height) / 2
return {artL - (cWidth - (item 1 of itemPosition)), artT + (cHeight - (item 2 of itemPosition))}
end tell
end convertPos