The Monster Boxes refers to the boxes the United States’ Mint ships the American Silver Eagle in.  In 1986 the United States started issuing the American Silver Eagle.  The American Silver Eagle is a one troy ounce 99.9% pure silver one dollar bullion coin issued as a numismatic coin for collectors and investors interested in securing precious metals.  The Silver Eagle Monster Box contains 500 uncirculated American Silver Eagles.  There are twenty-five rolls of twenty silver coins each. SilverCoins.Ag has a more thorough description of the Silver Monster Box.

Why Are Monster Boxes So Expensive???

...is what you might be wondering. A more detailed explanation is available at SilverCoins.Ag, but briefly quoted: "These boxes are usually kept sealed with the mint’s marking to insure the contents have not been tampered with as well as making transporting and storage easy. Dealers always sell bullion over spot price (market value) and these prices will vary from dealer to dealer; for instance if the dealer claims $2 a coin then 500 will mean an additional $1,000 per monster box.
If silver is selling for $30 per troy ounce then the market value will be $15,000, plus the dealer’s mark up brings the total to $16,000. Two dollars over spot is very low, $3-$5 is more realistic bringing the price of the example $16,500 at the bargain basement press and $17,500 is more likely. Looking at silver today at $33.60/ozt then 500=$16,800 then add $3/coin=$18,300.
A few dollars a coin/ounce can raise the price very quickly and some dealers will have extra fees. Shipping and handling may be accumulative for each coin or by weight, if this is calculated by weight the box and its tubes may also be figured in. Within the United States the purchase maybe subjected to a sales tax if shipped from within the same state. Also shipping from one country to another can also add extra fees."