nope there is no extra fee for that, sliding round is just for fill up the vacant seats of students who withdrawal there seats till 3rd round, 
there are basically 2 conditions for taking part in sliding round:

1) if u have paid the academic fees of rupee 40000/-
2) if u have freezed your seat 

if you fall in these two conditions so congratulations you are going for sliding round,

hope it would helpful for you!!!
"vese i am also going for sliding round this year"
good luck for your future!!!