Harriet Rubin's profile of CEO book collections includes the notion that "it is impossible to put together a serious library on almost any subject for less than several hundred thousand dollars." She also informs us with great reverence that "Mr. Leach has stocked his cabin in the woods of North Carolina with the collected works of Aristotle." Not to disparage the wisdom of the ancients, but the complete works of Aristotle are available as a two volume set from Princeton University Press that costs $80.75 at Amazon and is eligible for free SuperSaver Shipping.
It's not that impressive a collection. Indeed, it appears that you can secure the entire Loeb Classical Library for less than $10,000. This is, admittedly, a lot of money, but it's an awful lot less than "several hundred thousand dollars" and it would certainly constitute a serious library on a subject -- indeed, on several subjects.