What is the value of a Kennedy half dollar?

The value of any coin depends on its date, mint mark, and condition. Without that information, it's not possible to give anything other than a very rough guide:

Circulated JFK halves:

1964 halves are made of 90% silver and are worth about 35% of the price of an ounce of silver (see your local paper's financial page)

1965-1969 halves are 40% silver and are worth about 18% of the price of an ounce of silver.

1971-current halves have no silver at all and are worth exactly 50 cents. Feel free to spend them.

Uncirculated:

90% and 40% silver halves are on average worth about 55 and 35% of the price of an ounce of silver, respectively. Extremely well-struck coins are worth more.

1971 and later halves can be worth $1.25 to $4.00 on average, but again, very well-struck ones can be worth more.

There are also some more valuable exceptions but a professional would need to inspect the coins in person.

In circulated condition the coins are worth about $2.50 each; uncirculated ones retail for $3 to $3.25. However if the price of silver changes again they will fluctuate in value accordingly.

Half dollars minted from 1965 to 1969 are only 40% silver so they are a lot less valuable than their earlier 90% silver counterparts.

It depends on the year. A 1964 half is 90% silver and worth a little over $10. Halves 1965-70 are 40% silver and worth just over $4. Anything 1971-present, including the 1776-1976 half, is worth 50 cents.

To find the value of an individual coin ask another question structuring it like the one below(be sure to fill in the <> with the correct information):

What is the value of a <date> <country of origin> <denomination>