Since you would only have one relay to mount, you should not need another panel. Of course if you were adding a larger project, the savings would climb exponentially. In the past with relay control logic, safety circuit implementation was weighed out on a cost verses likely hood method. In other words, the basic emergency stop button, relying on operators to stop the machine before an accident occurred. Usually do to distractions and slow reaction time of humans, the button wasn't pressed until after the damage was done.
Safety curtains are about the most expensive item, but well under the cost of an accident.
Using the PLC to monitor safety risks is equivalent to having a full time employee watching, but with quicker reaction time than any human. The PLC logic should never be a substitute for hard wired safety circuits, just used as an enhancement to machine safety. Safety curtains should be ran in series with hard wired Emergency Stops. Always consult your safety equipment vendor and use a qualified Engineer when designing safety circuits. Scheduled and non scheduled.