Thats a very large deck you have there....

There is no real "calculator" you can punch in as every job is different, different ground, diff materials, for the 18k, what did they use? stirrups? cypress pine posts? is it screwed down decking, are you getting balustrading? etc.

To work something like this out, I split every piece of the deck up into peices, find out how much 'pieces' and 'length's' i'll need, then i add the cost all up.

First find out how many liner meters of merbau you would need. This is your most expensive part.

Find out how many posts/stirrups you would need

Stirrups/posts should be no more than 1500 apart.. (so get your width and length and cut it down to 1500mm squares..)

Bearers will run along the stirrups/posts, 140x45 treated pine, work out how many of them you'll need

Joists will sit on top of them, no more than 450mm apart, they are 90x45, work out how many of them you'll need.

Work out how many screw's you'll need. So divide your width by the board your using (say 5000mm divided by 140mm boards will give you 35 boards), each board will need 2 screws per joist, so multiply that by the amount of joists you have...

You'll need coach bolts to bolt the bearers into the posts/stirrups, Find out how many of them you'll need...

Once you know how much material you will need, you can just price it all up.

Factor waste (about 15% - depending how good you are)

Generally I've found, that when a tradie quotes for building a deck, if you half it, its your rough cost of material.....but if you need permits etc, then that cost will go up.

Buy the alan staines book. its well worth the investment...the weather is also pretty terrible of late...this may take you a very long time do if its a weekend project job...

Cheers.