Is there any specific amount or percentage to be paid as mobilization cost?.

The short answer is it depends. Construction mobilization is the effort that it takes to get a job off the ground. A lot of different activities may be considered moblilization. There is not a blanket answer as to what will constitute a construction mobilization activity. Yet, in many well-written construction contracts, mobilization costs will be specified in the contact terms or in the initial schedule of values. 

The simpliest way to ensure that mobilization is accounted for and ultimately paid is to set the costs out in the contract explicitly. This may be accomplished with one lump sum mobilization fee or a line-item list of costs. Having the cost of mobilization explicitly stated in the contract is always a good idea.