I know that there's a certain amount of work that goes into developing a driver that works on 1xAA voltages, but why is the price so much higher when you're getting less "light" overall?
You are perhaps comparing the Fenix E21, a 2xAA light where 'E' in the model number stands for 'entry level', to say the Fenix LD10 R4, a single AA light which currently represents state of the art for currently shipping lights when it comes to output and runtime off a single AA NiMH rechargeable or alkaline primary cell. (edit: I saw your clarification post only after writing this, guessed correctly!)
If that is indeed the comparison you are making, you'd be better off comparing the E21 to the LD20, both of which are 2xAA lights.
There are a number of differences between the E21 and the LD10/LD20. The E21 has simpler circuitry and for a 2xAA produces less output than the 2xAA LD20. The E21 uses a XP-E LED emitter instead of the XP-G found in the LD series. The LD20 offers more operating modes, where the E21 simply provides off, low and high.
No doubt the E21 was designed to fill a certain price point -- keep the light in the $30's USD, away from the LD series which is in the high $50's / low $60's USD.
The single AA LD10 has it's place. I prefer the single AA format for every day carry. It produces more than enough light for that purpose and for an amazing amount of time, all off a single AA in a light less than 4 inches long. For a simpler less expensive Fenix there is an E10, not generally available in North America but REI has them on sale from time to time (currently ~ 25.00, has been as low as 17 or 19 iirc). I don't think it fair hold up the E10 to the LD10 like we could compare the E21 to the LD20, because the E10 is such a different light -- Luxeon LED, 55 lumen output, off and on.
With the LD10 R4 and now the Zebralight SC51/H51 upping the ante when it comes to output and runtime, I'm hoping there's something of an AA driven light renaissance going on. Consumers will benefit.