During flat selection, you pay only option fee, ranging from $500 to $2000, depending on the flat type but you should have a valid HFE.
Payment for the flat will be something like -
Grant(s) + CPF + loan before / during key collection
If all these are insufficient to cover the cost of the flat, person pays the balance in cash
(not sure before or during key collection since no personal experience on this part yet 😝)
Chinese