My husband is a journalist and says the answer is simply not to pay them.
You need the people who speak to you to be free anyway, and not feed you answers you want to hear just because you are giving them money.
It doesn't really make sense to talk to the police in an area where prostitution is illegal, since they'll probably tail and arrest whomever you are talking - if not on the day of your interview, then on the next. Your interviewees need confidentiality and security and to trust you. Those things are the most valuable you can offer.
Getting in touch with prostitutes and their pimps isn't all that hard (at least where I am). You just need a bit of courage and to make it clear from the get go who you are and who you aren't.
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