Yeah, I was shocked when I first realized this as well, I originally thought that letters of course were more valuable than numbers. The price differential is due in large part to scarcity, there are only 10,000 NNNN.com's but over 450,000 LLLL.com's. There are even fewer NNNNN.com's than the LLLL.com's (100k v. 450k+). But it is also due to the fact that numerics get very good type-in traffic and actually have a lot of development potential. NNNNN.com's are much more likely to be developed websites than the LLLL.com's, in part because of their popularity as zip code websites and in part because the Chinese market mainly uses numerics because they have a lot of significance in Chinese culture and are easier to remember than the western alphabet letters. Numbers are also good domains because they represent dates, part numbers, words spelled on cell phone keybads (a big area for internet growth in the future), and many other uses.
You wouldn't believe the traffic I get on some of my NNNNN.com's, actually I can't even believe it (or understand it yet, I still don't know why so many people from China are coming to some of my numeric sites! ). Anyways, it has really corrected my bias against numeric domains. I am now a true believer that this is the biggest section for future growth in the domain market, largely because the western bias has undervalued these domains for a long time, and largely because internet use in China is going to boom over the next 5-10 years.
They may not be available to reg, but they are still great investments. Much more solid than many domains people are registering that aren't the .com extension in my opinion.