So with the half off sale most places I have looked the elixirs are half-off, but I found one way to get one where it isn't...
If you join a low-level dungeon with a high level character, you get that little flashing warning saying you don't get XP in this dungeon. You can click the (!) exclamation mark to be offered the "Gain XP In any Dungeon" elixir. When you do this, the elixir is still 5 plat. I am pretty sure I bought one, too.
