It is worth face value unless it is still in the original mint package.
25 cents.
The date is 1776-1976 and it's only a quarter.
Yes. There were US quarter dollars struck in 1975 but all of them carried the date 1776-1976. Likewise, all 1976 quarter dollars also carried the 1776-1976 date. There is no way to identify the coins struck in 1975 from those struck in 1976.
US$666.66
Only face value unless in mint packaging.
All US Quarters made in 1975 & 1976 have the dual date 1776-1976 so if the coin has a date of 1975 it's not a US coin.
Not a lot, and most people ask too much.
All bicentennial coins were minted in the years 1975 and '76.
In 1976, the US national average price of a gallon of regular unleaded gasoline was 61 cents - equivalent to about $2.34 per gallon in 2010.
The highest known sale price for a US quarter was $550,000.00 in May of 1990 for a 1901-S Barber quarter graded MS-68 by NGC.
Please check your quarter again and post a new, separate question. The first US quarters were minted in 1796. If you have a dual-dated 1776-1976 quarter it's one of the billions minted for the US Bicentennial, and is worth only face value.
I remember paying 1.39 in that time.