A professional appraiser can tell you how much the FMV is.
Put it this way, although there may be restrictions for selling 樓花 in HK (not sure about rules in HK), you can say it is worthless. There is a price someone is willing to pay to own the 樓花. The developer may have unsold units which can be an indicator. Similar completed units in the same area is also an indicator. Finally, you paid a price for the 樓花, the market price may have gone up or done after you purchased it. The real estate specialist in the area can figure out how much it worth as of any day/month.