As you can see from the graphic the value is $987 to be exact....at least that is what the fine folks at tweetvalue.com think.
I am sure the science behind this is pretty simple based on the number of followers you have with a slight adjustment for the number of posts you have.
Of course the real value of a twitter account should be based on the results of your tweeting. I am sure that there are thousands of users with the same number of followers as I have that have never gained a new contact or business opportunity from their efforts. Just as surely there are plenty of tweeples that doubled or tripled their profits via Twitter with only a handful of followers.
The power of twitter can really be seen when you post something interesting enough for others to re-tweet. Your audience can grow exponentially if you really strike a nerve with you audience so get out there and see how many RTs you can get!
As with anything else in the Web 2.0 space the key is paying attention and getting involved: monitor communications that mention your username and check the direct messages (even though those auto DM's are so annoying!) and answer their questions. If they like your answer you might just end up with another customer!