The commercial PDU you link to also supports current metering (for each port it seems), so it's not just a bunch of relays. Also (not included in that price) "Optional ENVIROSENSE temperature/humidity probe enables remote monitoring of in-rack environmental conditions and alarms for up to 3 user supplied contact-closure sensors."
Most of the rest are software features, you can get SNMP on arduino as well and probably whip up your own code to do "User defined power-up and power-down outlet sequences" or "Programmable load-shedding of less critical devices at user-defined intervals". And that PDU has a web interface too (not just SNMP), which you'll need code if you want that.
A non-trivial cost chunk goes into testing all that, warranty, etc. Also a 1U empty rackmout case will cost you [new] a little bit more than an arduino. Add the tooling to put all the stuff into that, etc.
You can look at this vid (and photos, PCB layout, schematic) to see what's inside an entry-level DYI networked PDU (no current metering).