That document says, "People often keep jars of mercury in their homes because they think that mercury is worth money, like liquid silver. In reality, mercury is nearly worthless, but the cleanup cost after a large spill easily can exceed the cost of a home."
http://minerals.usgs.gov/minerals/pubs/commodity/mercury/mcs... gives the price of mercury as US$1850 per 76-pound flask, up from US$1076 in 2010. That's US$54/kg, or US$730 per liter at 13.5 g/cc (cf. https://en.wikipedia.org/wiki/Mercury_%28element%29). Silver is currently http://www.cmegroup.com/trading/metals/precious/silver.html US$15.76 per troy ounce, which is US$506.70/kg or US$5315 per liter at 10.49 g/cc (cf. https://en.wikipedia.org/wiki/Silver). Silver also has the advantage that it won't make you sick if you spill it.
As a point of comparison, copper is sufficiently expensive that people steal copper pipes and wires from unoccupied houses. Copper costs US$2.41/lb. (US$5.31/kg), less than 1% of the cost of mercury. The junk man in the slum where I'm going this afternoon will pay you AR$35/kg for it, which is US$2.23/kg, which is enough to incentivize the cartoneros to break the yokes off any CRT discarded on the street within hours. (He doesn't list a price for mercury on his sign.)
So "nearly worthless" seems like an exaggeration — mercury is worth more per liter than any other commodity substance you're likely to have in your house other than silver and Freon-22 — but it also doesn't seem like a really smart investment, especially if you have kids.