Earlier missions put silver at a concentration of 100 parts per billion (ppb). The LCROSS mission found a possibly higher concentration, but the original publication (subscription only) does not contain an estimate of the concentration. In fact, the article only says this:
This (spectral lines found in images of the crater) suggests a concentration of volatile Na (and perhaps Ag) near but not on the surface.
It appears that the popular-science press has been hyping this up beyond what's supported by actual data.
Even if LCROSS found a higher concentration of silver, that measurement is only valid for the impact site. It'd be very difficult to extrapolate this to the entire Moon. On Earth, the concentration of silver and other materials varies wildly, and it's only high enough to economically mine the silver in a few places. To give an idea, one silver mine has a silver concentration of 0.3 %.