Than and Then

I have a pet peeve about the proper use of some words in the English language. For the last few years it seems that I see more and more improper use of the word then. I have seen it from all sorts of people - people with Master's degrees and people with less education than that. We landed in Bozeman, MT today and went to the Avis rent a car location. Posted on the front door of their office was a note stating, "Less then $50 cash on premises". It curls the hair on the back of my neck. I don't know why. I know it is a trivial matter but still it drives me crazy. "Than" is supposed to be used when comparing - less than, more than, I would rather go here than there. Then is supposed to be used in reference to time. For example, then we went to the store or then I left. While I haven't gone to the dictionary to give the actual definitions I think you get the point. So why do so many people use the word then the wrong way.