Well, it is not that there is no prostitution in Belgrade, there is, and it is quite large and growing. However, as it is illegal and the police are trying to cut it down, it is hidden from tourists.
So, you could try international sites. You can still find hot Russians including more services at cheaper prices than in London. However, you are not going to find bargains in Belgrade. At least not without the help of someone local and assuming some risk of being raided, and fined.
Nightlife in Belgrade is awesome, bistros and drinks are cheap, and Serbian girls are very hot and approachable. So, you can still enjoy Belgrade.