Aren't you looking for cheap? 'Cause Dark Eldar ain't it. You need boats. And that's not cheap.
Necrons? Well, I don't like them. And, certainly at this point in time you're not going to be a unique snowflake for getting them. But there's no real reason not to get them. Although it's certainly seeming that vehicles, too, are invaluable in a Necron army. And so that means 'not cheap' either. If you had gone for Necrons, say...Three months ago, they would've been the cheapest army around and you would have a leg up by now...But, time's change.