Denominations can be as small as 1 cent. The highest face value I have seen is a $200 tax stamp used for paying the transfer tax on machine guns. A standard 1st class stamp is 49 cents.
Proper valuation will require a more detailed identification. Consult a postage stamp catalog, Gibbons is a standard in Great Britain. It should be available in your local library. It will help you determine the specific stamp and provide a value for you.
As of May 11, 2009 standard postage went up to 44 cents.
$0.20
.75
Today (February 4, 2015) it is worth 80 cents US.
Worth 6 cents
The lowest value US stamp was the 1/2 cent. A couple different versions were issued.
A US first class stamp dated 2002 is worth face value only, and it's still valid now.
8 cents.
The US issued a Chester W Nimitz stamp.
39 cents
I have an old paper food stamp and i was wanting to know what it would be worth now. It's a 1999 $1.00 paper stamp. It's in great shape.
Air mail stamps are good for ordinary postage- your stamp is still worth 7 cents if unused.
50 cents if unused. Nothing if used.This must be the most common US airmail stamp.