Here's Northwest's policy:
Excess Valuation Passengers may declare a higher value for checked luggage, by purchasing additional liability insurance at the time of check-in. Charges shown below are for one-way travel.
Domestic/Transborder: Per Passenger
Maximum Amount -- US Dollars $5,000.00/Canadian Dollars $6,750.00
Charges: US Dollars $1.00 for each additional hundred dollars of value Canadian Dollars $1.35 for each additional $135 of value
Example: If a customer wishes to purchase an additional $500 of liability insurance, and they are traveling domestically, the charge for this would be $5.00
International: Per Passenger
Maximum Amount -- US Dollars $2,500.00/Canadian Dollars $3,125.00
Charges: US Dollars $1.00 for each additional hundred dollars of value. Canadian Dollars $1.25 for each additional $125 of value.
They also explicitly say:
Customers should NOT pack medications, car keys, tickets, passports and other legal documentation, or valuable items such as jewelry or cameras in checked luggage. For a complete listing of items excluded from liability, please refer to the full text of the Northwest Airlines contract of carriage, which can be obtained online, at a Northwest airport counter or ticket office or by writing to: ...
And their contract of carriage excludes liability for:
1) MONEY 2) JEWELRY (INCLUDING WATCHES) 3) SILVERWARE 4) NEGOTIABLE PAPERS 5) CAMERA EQUIPMENT, FILM, PHOTOGRAPHS 6) PHOTOGRAPHIC EQUIPMENT 7) ELECTRONIC EQUIPMENT 8) SECURITIES 9) BUSINESS DOCUMENTS 10) SAMPLES 11) COMPUTER EQUIPMENT AND RELATED COMPONENTS 12) PAINTINGS 13) ANTIQUES 14) MANUSCRIPTS 15) IRREPLACEABLE BOOKS OR PUBLICATIONS 16) KEYS 17) MEDICATION 18) IRREPLACEABLE ITEMS (ONE OF A KIND)
Looks to me like their "insurance" is pretty worthless considering all the exceptions.