Right on dot! This extension would not attract flippers to make good offers and would be taken up.... only by an enthusiastic entrepreneur who would look to build a site on it. You would have to wait for such a buyer to come by... to get exceedingly good price! Regards, RightMan
You can use the .mobi sales report to get a good idea on the value of 3 LLL .mobi domains. You can find the .mobi Domain Sale History here.
I know someone who rocks and was kind enough to actually give me a lll.mobi which I've since developed my music retail back end on. Check out http://mobility.mobi/ it's an awesome place for all things .mobi and mobile web related.
Recent sales would suggest they are worth registration fee. But with decent letters you might get mid $xx. I agree with posts above though, .mobi is much more a developers game than us domain flippers.