The US has issued a single clown circus stamp and a set of four 29 cent stamps. The 5 cent stamp is Scott number 1309 and the First Day cover is cataloged at $2. The 29 cent set is Scott number 2750-2754 and a First Day Cover with all four of them is cataloged at $3.
A "Harry Potter and the Sorcerer's stone," first edition, American edition, without the dust jacket is worth between $5.00 and $20.00 in the United States.
25 cents.
First Day Covers are worth more than ordinary covers. Stamps bought on the first day themselves, without cancellations, cannot be identified as being issued on the first day. Leave them on the cover if you have them!
Alot
In perfect condition, it is worth 25 cents.
45,000,00
A First Day Cover on the official Teddy Roosevelt envelope is worth about $7 or $8 depending on condition. If it's just a stamp on a plain cover, then it's worth about .50c. The stamp is worth .20 used of envelope.
one hundred dollars
$7000 to $10,000 depending on condition
Commercially produced First Day Covers of standard commemoratives like the Challenger are typically worth a dollar or two. They can be purchased for on eBay or through dealers. Higher value stamps, or specific cachet makers are worth more. A specialized catalog will list the value of a cachet or un-cacheted cover.
That is Scott number US 2853. It has a value of about $2 with cachet and no address.
If you have a first pressing and the entire outer cover is intact, you can probably get about $15US for it.