Try something like:
If [ not IsEmpty ( Quotation::quotation_id ) ]
Show Custom Dialog [ "Quotation already created!" ]
Else If [ Costing::status <> "Complete" ]
Show Custom Dialog [ "Cannot create Quotation. Costing is not completed." ]
Set Variable [ $jobID; Costing::job_id ]
Go to Layout [ "Quotation" ]
Set Field [ Quotation::job_id; $jobID ]
BTW this assumes that you have your relationships setup properly.
Thanks a lot to you Mr. Vodka.
That solved my problem.