Anything from $0 to $xxx,xxx. Just like there are dictionary words in the .com that are still available & are worth $0, so are in the .mobi extention, evey more so with .mobi I would research what type of sales are happening in .mobi to get an idea of what names might have value, don't just rush out & buy names that are crap otherwise you'll waste a lot of money.
.MOBI domains are selling for very high prices right now, but personally I expect mobile phone browsing technology to improve enormously over the next few years to the extent that there will be no need to "water down" websites for them, hence making the need for a .MOBI site sites much less. My advice would be if you register a domain, intend to sell it on fairly soon and not to sit on it for ten years waiting to it to appreciate in value. In terms of an appraisal, I agree with htmlindex: could be anywhere from zero to six figures.
I tend to agree with you. These days what sell more is the 3 character names in the mobi section @ sedo. I do feel non plural names like length inch will have a buyer someday . I think almost all the good ones have exchanged hands at the mobi auction.