That is strange! I thought for EU countries you don't need to pay extra for a green card, they issue it for free and it's valid for Serbia.
take a look at this too, might be interesting.
http://livinginbelgrade.com/news.php?idNews=74
I came across that site and I found it very interesting.
Thought to share with everybody.
http://LivingInBelgrade.com
Though meant to be for foreigners living in Belgrade, you can find tons of useful information also for visitors and travelers.