The estimator has been known to be way off. I belive it takes into account daily budget, your ad placement, and average CTRs of how others have done with that keyword.
Really, I'd heartily advise that you not let the traffic estimator scare you away from using a keyword you believe in, because it has given you a low click estimate.
IMO, just use the keyword, and watch your stats for a few days until you can judge what happens in the real world. Your actual stats will always be more meaningful than an estimate.
Also, please remember that the estimator has no way to judge how well targeted your keywords and ads are, nor how well written the ads are. These two factors largely determine the number of clicks you'll get per day.
To return to my silly example from the past: if you run on the keyword 'cat' (which the estimator would predict you'll get a lot of clicks for), and then show an ad about Dogs - well, you probably won't be getting too many clicks.
AWA