I think that the upgrade fee for park hopper option is a flat fee though, of more than $10. Also, I have read that even if you buy a discounted ticket,
disney will only charge the upgrade from the gate price of your ticket, absorbing the cost from your discounted price to current gate price. I believe I read that from Chesire Figment who seems to be a reliable source as far as ticket info goes, but I am not positive. Hopefully someone with a firmer idea of upgrade costs can post here and clear things up for the OP!