A weapon cannot be an epic weapon and at the same time not an epic weapon.
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Let's look at this issue from a DM's point of view. It is generally not a good idea to allow PC to evade rules, such as getting something without paying the full price. It is always better to say no to them when they are making their character or purchasing gears, then to have no undead encounter or make changes to rules after you realize it is actually over powered.
Epic, in a sense, is very much like the Burning Crusade Expansion. You raise the price to make sure PC have a way to spend their hardly earned money from previous adventures, and Epic DR for PCs to quickly switch to a new set of gears.