Cost to GC of using weak references in C#?

In another question, Stephen C says:

A second concern is that there are runtime overheads with using weak references. The obvious costs are those of creating weak references and calling get on them. A less obvious cost is that significant extra work needs to be done each time the GC runs.

So 开发者_开发知识库what exactly is the cost to the GC of a weak ref? What extra work does it need to do, and how big of a deal is it? I can make some educated guesses, but am interested in the actual mechanics.

Please check Jeffrey Richter's article about Memory Management in .NET it must clear up things a little.