Cost

I believe an estimate to how much money was needed during the convict migration was very little as barely anyone was paid. Convicts that were set free wouldn't have been given a small amount of money as they were given land, housing and animals to start up a new life. I believe that back in the day, without currency, all people could do was trade. If something was wanted then an item of particular interest would be traded with another person. 

As written in "Australian Early Currency"

When Captain Arthur Phillip, Governor of the new settlement, arrived in Sydney Cove in 1788 with eleven ships and 1,487 people, including 759 convicts, the small colony was virtually penniless. Apart from 300 pounds in currency held by the Governor, the only coins available were those carried in the pockets and purses of the passengers. It was not forseen that the small colony would almost starve to death because of drought, poor soil and various other problems which made it impossible for the settlement to become self-sufficient.