There is also a website called twitter grader. Klout, twitter grader, whichever of these tools you use to evaluate a twitter account, it's really not an accurate way to judge an account. The only true way is looking at each follower's profile, spam or not, and see how much real interaction with related followers they have - the previous comment about these sites stroking one's ego is 100% correct.
Not sure how accurate this one is, but it tells you what percentage of following is fake or inactive: http://fakers.statuspeople.com/
Try and sell it. Create a starting bid for it and offer it in a post on an Internet marketing forum. The reaction to your starting bid will give you an ideas of where about the true value resides. If you propose $500 and people laugh at that, you know it's probably not going to sell for that. Where as if people willingly offer that, you know you have something... carry out a would-be auction but then decide not to sell. This is an instant way of knowing how much people really would pay.
Since selling a Twitter account is against Twitter's TOS, I would not recommend doing that. Of course, if you sell a business and/or a website and the Twitter account comes with it, it would be different. I know on this forum for example selling Twitter accounts is against the rules. Sponsoring tweets however is ok with Twitter. Here is one service that has been around for a while and a lot of high profile users/celebrities are actually offering this: http://sponsoredtweets.com/ Chloe Kardashian will tweet you for 13k: https://app.sponsoredtweets.com/tweeters Kim wants you to call. I would recommend going with the 4,000 - 5,000 follower accounts and take my time to find the most targeted one. Better deal and probably very effective. (If you manage to find one where the pattern of tweets, topic etc. somewhat match what you have to offer).
I have a Twitter account with roughly 2000 followers that was evaluated at $1,800. (It's related to a niche website.) I doubt I'll find a buyer willing to pay that price but it's a good feeling to have such a valuable asset...
The value can actually be even higher if those are people that came FROM your site TO the Twitter account. But I would doubt that it would have the same value to anyone else, unless that person bought your site, took over the Twitter account with it and continued doing what you're doing.
I would recommend you to try Klout, or Twitter Grader, With Twitter Grader you can see your profile rank on Twitter.