I don't say you to use old menu, i say to use new menu but forget the way key is retrieved (tutorial is really good and helpfull but that part should never have been put in it i guess).
So just pass money amount in item info like you do, but don't try to get the key with that data because obviously data doesn't contain key but money.
So use item instead of key.
key : 1 to 9 (or more)
item : 0 to 8 (or more)
MENU_EXIT, MENU_NEXT and MENU_BACK (if names are correct) are all negative values.
If you don't understand what was said in that post and in previous posts, than you should stick with more basic plugins.