The problem is that there is a very good chance that the power cord will be dumped in favor of another.Wow, I really did not expect that as a response. In that case why supply a power cord at all with the amp? Speaker dont come with speaker cables. Heck, why supply tubes, those will most likely be dumped too.
Regarding the mark up, I did some research and in general I would agree, it would be difficult put together a cable for less than $100. However, there are medical grade power cords available for about 20 to 25$ from brands such as Volex or Qualtek. However, they are only available in 14AWG to 18AWG with 15amp IEC connectors. 12AWG comes with 20amp connector. So for most audio components to you would be stuck with 14AWG. So it would appear that cables such as the Venom 3 probably do over a good value.