So far, I have been unable to find any estimates that reach the 50-90% figure quoted.
The Eighth report of the Dutch National Rapporteur on Trafficking in Human Beings (2010) explains the difficulty:
There are conflicting estimates of the scale of human trafficking from different organisations and countries. It is therefore very difficult to be certain about the scale of the problem, which underlines the need for the systematic and coordinated collection and management of data.
Since human trafficking is often hidden and victims are often unwilling or afraid to speak out (or do not
realise that they are victims), there are probably a large number of unknown cases of human trafficking
(a large ‘dark number’). Consequently, statistical trends based on the number of known cases of human trafficking
usually do not directly reflect developments in the total number of cases of human trafficking.
The number of known situations of human trafficking depends to a large extent on factors such as the public
attention for human trafficking, the priorities of the investigative services and the public prosecution
service, the method of registration employed by victim support organisations, and changes in the law.
However, they do have a detailed analysis of the reported cases of Human Trafficking in the Netherlands, and the numbers are disturbing, with over 900 victims reported in 2009.
Of these, at least 45% were being exploited in the sex industry (likely a lot higher, given the significant "unknown" figure):
Obviously, the unreported crime figures are likely to be much higher. These numbers should be compared with the number of prostitutes in the Netherlands - Wikipedia cites a number of different studies that range from 15,000 to 30,000.
As for consent, the 2004 version of the report includes figures on how the traffickers gained compliance: