I assume (a), since that is the only reasonable assumption based on context and real-world knowledge.

As for the snobbery, though, this isn’t a matter of merely acceptable usage. This is both correct and accurate usage, and has been for nearly a millennium. Noah Webster himself puts in his Dictionary for Primary Schools that over means “more than” (p. 201), so if Webster’s current dictionary disallows that, it must have been after Webster’s death. Given how often “over” is used to mean “more than” in American English, a dictionary that doesn’t include some mention of this fact, not even to condemn it, seems neither accurate nor authoritative on American English usage.

“Now, a brief historical note: During the American Civil war, apparently it was quite common for under-aged boys to falsely swear, ‘I’m over 18′ so they could be in the army. The story goes that quite a few of them wrote the number ‘18′ on a slip of paper and put it in the bottom of one of their shoes prior to arriving at the recruiting office, thus technically validating their oath” (Betalogue - http://www.betalogue.com/2004/01/13/more-than-vs-over/).

To me, using over to mean more than is vague and allows for misinterpretation, which is unacceptable. If you have the words to say it best, why wouldn’t you?

If it’s a matter of untrained minds misusing it, then Jane Austen’s an untrained mind.