Hello brothers and sisters

I am presently building a 3 storey building and need your advice on the cost of decking for each level to compare with what I have been spending so far as I have a feeling that I am being ripped off.
The building is on the second decking and I was quoted 4.5 million Naira. Each level has 7 rooms. Meanwhile I cost me that much to complete the first decking. It’s quite hard to establish what’s going on as I live in Australia but there’s this feeling that I’m being ripped off. I’m just here to find out the truth from honest people who have built this type of house. I was also told it would cost 18 to 20 million to complete the entire building. Please advise