They most likely have no value above melt. As they are not coins, they have no legal tender status.
If deciding on an appropriate value make sure you look at the actual weight and (IMPORTANTLY) the percentage of silver in the round. A one ounce round of 100% silver is worth about $15 depending on the spot price. If the round is the same composition as most US silver coins (90% silver, 10% copper) then the melt value of the round is somewhat less.
Any value above the metal content is based solely on supply and demand. While some of these rounds are actually in demand most are not.
Also be aware that many of these rounds are actually silver plated rather than a silver alloy. This makes them worth mere pennies as a metal store.