Here is the link to the USPS site where Registered Mail is discussed. Note that the only way you can ship money and get insurance is with Registered Mail. It's also interesting that items of "no value" (like documents) can be sent without declaring a value.

http://pe.usps.com/text/dmm300/503.htm#1100045

I've found that it's generally cheaper to ship merchandise valued below about $1100 by insured First Class mail. Above that amount, Registered Mail with a declared value is cheaper.

The Postal Service almost never "loses" a Registered item. It is rarely lost to theft, but most often lost to 'crash and burn' of the vehicle carrying it. Basically, you don't have to worry about safety if you send something Registered.