What is the silver content of us commemorative silver dollars?

Nearly all commemorative silver dollars have been struck in an alloy of 90% silver and 10% copper, the same metal that was used for circulating dollar coins up till 1935.

Silver "eagle" dollars issued since 1986 are made of 99.9% silver but these are sold for investment rather than as commemoratives.

One of the few exceptions would be Bicentennial dollars dated 1776-1976. Special collector versions were made of clad 40% silver. However these aren't strictly commemoratives either, because the same design was used for circulating copper-nickel versions.