That is very misleading. That is only a pre-order. These boards won't be available for at least a month or so yet, and the price IIRC is around $400.
Yeah, they're not actually that much cheaper than the equivalent full dev boards and your ability to develop other stuff for them is vey constrained.
I'd suggest buying a DE0-nano if it wasn't for the fact that they've cheaped out on the power circuitry to the point it's very difficult to get reliable mining at any speed, let alone max the chip out. If they'd used more efficient power regulation circuitry it ought to top out at about 25 MHash/sec, probably even with just USB power, which isn't bad for the price.