Sending a monster to the Graveyard for "Altar for Tribute" is a cost.
You can select a Token Monster as the cost for "Altar for Tribute", but you won't gain any Life Points because Tokens are taken off the field instead of going to the Graveyard.
You can send your opponent's monster that you control to the Graveyard as the cost for "Altar for Tribute", and you would gain the Life Points.