What is the value of a 1776-1976 solid silver quarter?

Answer 1

The US has never made solid silver coins for circulation. They'd wear out almost right away - they contained 10% copper.

The only silver quarters made in 1976 were special 40% silver collectors' coins that carry an S mint mark. As of 03/2010 they're worth anywhere from $1.50 to around $12 depending on their quality.

All other 1976 Bicentennial quarters were struck in copper-nickel. These have a D mint mark (Denver) or are blank (Philadelphia) and are only worth 25¢ if you found one in change.

A number of private companies plated normal circulation coins with a small amount of gold or silver and sold them as "collectibles" but the market for them pretty much evaporated once the Bicentennial celebrations were finished.