Purchasing Requisition Hierarchy

 hi Expert i am involve an amazing kind of case while making the Purchase Requisition Hierarchy ..

Actually i Have to make the Requisition Hierarchy based on the 5 levels as, 
1) CEO
2) HOD 
3) Manager

4) A.M 
5) User 

its seems easy up to yet but now the requirement of user is that if User Enter the Requisition so its get approved while manager gives his approval mean at the third level well if manager enters the Requisition so it get approve when CEO gives his corresponding Approval now u could say we can do so by making the appropriate approval group with corresponding approval limits and assign them with respective Position but at our client when ever we either user or any one else enters the Requisition we has to enter the Unit Price is to zero(0) ...!!