Dear Student,

The seats which will be still vacant after 1st and 2nd round of counselling will be transferred to mop up round.The candidates who don't get any seat in 1st and 2nd round will don't need to pay any registration fee for mop up round but the candidate which is registering himself/herself fresh in mop up round and don't participate in 1st and 2nd round will have to pay and there is no specific strcuture it will be displayed to you it may be normal fees like any registration 1000 for UR and 500 for SC/ST/OBC.For security fees(refundable)  it would be 10,000 for UR or general category and for reserved it is 5000.You can read here

https://medicine.careers360.com/articles/neet-counselling