public class DocumentDetails
{
    public int Id {get;set;}
    public int TypeId {get;set;}
    //Прочие свойства
}
public class AddDocumentItem
{
    public int DocumentId {get;set;}
    public int MaterialAssetId {get;set;}
    public decimal Amount {get;set;}
    public decimal Price {get;set;}
}

public class AddDocumentItem
{
    public DocumentTypeInfo TypeInfo {get;set;}

    public int DocumentId {get;set;}
    public int MaterialAssetId {get;set;}
    public decimal Amount {get;set;}
    public decimal Price {get;set;}
}

public class DocumentTypeInfo
{
    public bool HasPrice {get;set;}
}