If the quarter is in circulated condition, it is worth around $0.25. However, if the quarter is in uncirculated condition, it can be worth significantly more. For example, a 1964 quarter in MS-65 condition is worth around $25.
There are also two different varieties of the 1964 quarter. The first variety has a small "D" mint mark below the eagle's right wing, and the second variety has a large "D" mint mark. The large "D" variety is more common and is worth around $0.25 in circulated condition. The small "D" variety is rarer and is worth around $1 in circulated condition.
Ultimately, the value of a 1964 quarter depends on its condition, variety, and overall desirability to collectors.