How much to buy a 1972 American Silver dollar?

Assuming you are describing an Eisenhower dollar, a cupronickel* circulation coin would sell for face value or slightly above. A proof version of the coin would sell for 2 or 3 dollars.

(*) Even though they are called "silver" dollars, circulation $1 coins made since 1971 have all been made of either cupronickel or manganese-bronze. They do not contain any precious metals.