Considering you have basically all of the Diecimila's components on a four layer board with double-layered assembly, I can see there's a noticeable cost increase.
Assuming parts costs are the same (which based on the volume of Diecimilas compared to Nanos, I think it's safe to say they aren't). You've basically doubled the PCB and assembly cost (two runs through the pick-n-place, two runs through the oven).
If Gravitech wants to sell their (excellent) 4-layer Nano design for $50, why not? At the same time, others can sell an almost identical two-layer design for $25 or $30.
I think this is exactly the right way to look at things, there are a whole host of options, depending on what you want to buy.
Gravitech sells a pre-assembled, stamp-sized, official, Arduino for $50, that's one option and a pretty good option at that. Other than (possibly) price, it's a win on all aspects.
Ladyada sells Boarduino USBs for $25, they're bigger, kitted, and unofficial.
I sell iDuinos for $18, they're bigger, kitted, unofficial, and not listed on the Arduino.cc 'Hardware' page.
Paul sells RBBBs for $11, they don't contain USB, they're kitted, and unofficial.
There was a post recently about a clone stamp sized board with onboard USB.
How many times has there been a comparison to the Basic Stamp that IIRC is priced at $50 and has always been railed for being too expensive.
And now, you can actually make the comparison properly. I don't see anyone rushing to replace the Diecimila with the Nano as the 'lead Arduino' board. If anything, this is a replacement for the Mini, in which case it's far more of an upgrade than a dramatic change.
Depending on what you want, you do have a range of options. The Nano is an excellent option if you need/want any of it's aspects and I think once you start imagining they're costs, you can see how $50 is a reasonable price point.