I can give a few "possible" answers to this question. My post gives food for thought for both dot.com buyers and sellers.
There are "certain" individuals/companies who troll popular keywords and phrases. They then "buy up" popularkeyword.coms using those words/phrases. In turn, they re-market these sites to individuals at a seriously inflated price (sometimes making thousands) to someone who searches for a particular name, finds it and really, really wants it.
Example: If you just HAD TO have the name JohnSmithInBrooklyn.com and it was already owned by Blahblah Jones (in the U.S., Scotland or even Africa for that matter), you would have to purchase that name to use it or risk a copyright infringement and possible legal action. This can happen EVEN if you are the ONLY John Smith In Brooklyn that exists! The shysters who re-market and recycle these names count on your ignorance of these matters.
In our "fad driven" society (and at any given moment) one key word or phrase may become outrageously popular and would thus produce almost immediate traffic from a pre-determined audience. Often, those people have nothing more than an "under construction" notice on the site and have no intention whatsoever of ever actually using it. They'll wait for some sucker to come along and beg for it!
Sometimes, they slap up a site with just a few reputable and useful articles based on popular key phrases, tell the bots it's static content which will benefit the public, and then they're an instant hit! Initially the traffic may be unbelievably high! As visitors return, however, traffic WILL dwindle if the site is not maintained. So there you have your prized domain name yet it is or may already be on the decline (based on the time of ownership AND the number of owners of that particular name).
From a purchaser's standpoint, the question is, do you know where the traffic comes from and/or "who" is supplying the figures? Are they credible sources? What makes you think so? If not, run, don't walk to another domain name. From a seller's standpoint, don't underestimate the purchasers out there now. Otherwise, you may pay for something that is worthless in the long haul or never sell your "priceless" site.
After all, if someone already thought of it, you are already passe. The lesson is, if the dot.com you want is already out there, you need to get more creative! Savvy, informed designers and/or businessmen/women will check these things out and decide for themselves what your dot.com is worth TO THEM. Even smarter people don't purchase a dot.com which already exists! There are, of course, extremely rare circumstances which are an exception to that rule and only you know if you are in that category.
An instance of "passe" would be the Demo Content on every Joomla install. Some will start out with that to get some exposure, others just to use while they learn the ropes. Either way, do a keyword search on that content, see how many sites are listed and where they rank.
Joomla.org is #1, of course, and then there are millions which come after. If you aren't Joomla.org, you'll probably never rank unless you are creating good quality content that they don't carry. Other than Joomla.org, chances are your content won't get many hits other than newbies, loyal followers or incidental passers-by.
When a site is ready for production, it's wise to remove that content or it will actually lower your rankings. Even though it's of value to the public, you are using "common" content.
That being stated, you may (but probably won't) get high traffic on any site based upon selected key words/phrases, SEO standing, etc. even when there is no content! But don't kid yourself, bots are now sophisticated enough to know when you are "cheating" in this arena. (Note: If you don't know what a "bot" is, stop here now, go to Joomla.org and look it up!)
There's a fine line, however, between what would actually "produce" a continual flow of traffic and that which is popular for only a moment. I believe it's referred to as a "flash in the pan" syndrome.
I believe in the philosophy, "There is no new thing under the sun...." A site's popularity ebbs and flows from moment to moment on the web. In the end, there is simply no "gimmick" to your site's value. Unless someone has access to your "actual" stats and logs on your server (God forbid!!!), they don't have a clue what it's worth!
Sheer perseverance is the major reason my site has survived but not without cost some months, a pittance of profits the next and a few "windfalls" during peak months. I have ups and downs and all arounds on traffic based upon whom I inform, how, and where I post notice that I have a new article. In short, I have long since quit looking at my stats and tried to focus on putting something of quality out there. If I have done my best, then and only do I feel a sense of accomplishment.
Bottom line is this: unless you are BBC, Fox, CBS, Mayo Clinic, Geico, (or RocketTheme <chuckle>) your site won't be "worth" much if you aren't aligned with some "buzz word," you don't have an established following and/or you do not have a solid reputation on the internet.
The better questions would be:
How much is your site worth to YOU?
Is your "product" unique to you or are you just another internet storefront?
How far are you willing to go to be the best at what you are attempting to accomplish?
Are you truly "unique" or just another "fly by nighter" trying to cash in on the latest "internet fad"?
HOW is your name unique to you?
How will you establish your brand name?
How long might that take?
Dependent upon your answers, it's a totally "subjective" valuation. Any valuation on your site will be subject to a subjective valuation and precisely who will determine the one performing said valuation is an expert?
Whew! I should have put this on MY site this morning.
P.S. Whaddya' know. I just did.