Bringing in wall mains will not be less. 5V wallwarts are really inexpensive these days.
Use inexpensive panel mount jack to receive power:
http://www.taydaelectronics.com/hardware/dc-power.html
http://www.dipmicro.com/store/index.php?act=viewProd&productCode=DCA-0510
Need to ensure have mating adapter.
Keep the 120V/240V away where someone else has taken care testing & certs.