{!REQUIRESCRIPT("/soap/ajax/9.0/connection.js")}

var count = '{!Contract.Count__c}';
if(count > 300)
window.parent.location.href = "/apex/page1?id={!Contract.Id}";

else
window.parent.location.href = "/apex/page2?id={!Contract.Id}";

<apex:page standardController="Contract" extensions="myControllerExtension" action="{!Redirect}">

public class myControllerExtension {

  private final Contract Contract;

   public myControllerExtension(ApexPages.StandardController stdController) {
      this.Contract = (Contract)stdController.getRecord();
   }

   public PageReference redirect() {
    //Query with necessary Id for your field
      Contract = [Select Id,Count from Contract limit 1];
      if(Contract.count>300){
        return pagereference ('/page1?'+ContractId);
      }else{
          return pagereference ('/page2?'+ContractId);
      }
   }
}