The crucial point here is that counters are objects that function as tokens; they represent money, but are intrinsically worthless. Noah Webster defined "counter" in 1828 as:

"1. A false piece of money or stamped metal, used as means of reckoning; any thing used to keep an account or reckoning, as in games."
http://1828.mshaffer.com/d/search/word,counter

Counters are used in games, like chips in casinos, for example; the chips represent money but are worthless until they are cashed in. So it is with counters; they represent money but are not the real thing.

This is the point of "box of counters" here. Someone who is more concerned to gain possession of a box of mere tokens than of a box of title deeds, which have real value since they confer possession of real property, is "indiscreet" (foolish).

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Note added at 1 hr (2012-02-28 10:34:28 GMT)
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The point of the metaphor in context, of course, is not to advocate devoting one's attention to acquiring wealth, but to warn against attributing value to a representation or symbolic token of the truth rather than to the truth itself.