Does that mean that PDUs is much more cost effective as compared to plugging it directly to the wall?
I think you are missing out on the reason for the PDU. If you had a wall plug for each psu (with proper voltage and amps) you would not need a PDU. This is more common in say 110-120 volt. But a PDU allows you to take one big power source (via the outlet) and distribute it to many item's. For example say you have 240 30 amp running to your miner's most likely you will need a PDU to take it from the outlet to your PSU's.
It's more about being a necessity in some setup's then the cost aspect. But you could save money running 1 wire to it vs many lower volt/amp wires (but this could very).
I suggest reading post before:
I finally understand it now.. So it is much safe to be using a PDU as compared to running it from the outlet.
1. PDU offers extra protection
2. save cost as you would only require 1 outlet from wall as compared to lots of outlet installed.