Well, a typical textbook from West or Aspen, which most classes use, runs about $150.
Then you have other materials; treatises, study aids, course packs, outlines, supplements, etc. Prices vary wildly depending on what you buy. Some of these, like the hornbooks, are just as expensive as your textbook. And since the majority (or at least about half) of the courses offered after your first year's required courses are only two credits, it's entirely conceivable that you could be shopping for books for six classes.
I'd say $5,000 is definitely towards the upper limit of what would be necessary. I've never come all that close to $5,000 in required books, but picking up, say, five textbooks, a couple treatises, some Q&A books, and a nutshell or two (or something equivalent)... that'll put you well over $1000/term. A good number of my classes have had more than $600 worth of books, if you picked up all the recommended materials.
Of course, I should've pointed out that I was considering software for laptop exams (which you need to purchase every year) and other supplies (e.g. the highlighters I go through by the case) in that figure, too.
Overall, $5,000 is a high estimate, and I personally spend less, but it's not unrealistic.