The cheapest way to program a plc for manufacturing is to program it correctly , make it as safe as possible, leave spares for I/o, document your work for your customer, and meet the requirements of the job.
Why
Do it right the first time and get a good reputation.
Taking shortcuts will get someone hurt or killed and result in lawsuits and guess who gets named as one of the defendents - You.
Having to go back to fix problems may mean you have to rebuild the entire control panel and you will loose a customer and eventually everyone will know your work isn't worth a dime.
I have seen several companies go out of business by taking shortcuts like the ones I mentioned.
regards,
james