Scott # 999 is green and shows Carson Valley, value is $.04.
Worth 6 cents
The lowest value US stamp was the 1/2 cent. A couple different versions were issued.
8 cents.
A 28 cent stamp that is unused can always be used as 28 cents worth of postage. If it is used, it probably has a minimal purchase price of fifteen cents from a dealer.
No, it is only worth 32 cents.
I bought one for about 20 cents.
3 cents
4 cents.
Air mail stamps are good for ordinary postage- your stamp is still worth 7 cents if unused.
That is Scott number US 2418 It has a value of 1.30 mint and minimal 15 cent used.
Not sure what you define as an original stamp. A US 3 cent unused stamp is worth 3 cents toward postage. It may have more value as a collectible, but you would have to consult a stamp catalog to fully identify it and get an estimated value. You might try the online version at Mystic Stamps as a starting point.
There is a lot of variety in asking about a 'two cent stamp.' You will have to determine a lot more about it to determine value. Consult a postage stamp catalog, usually available at your local library for exact identification and catalog values. Most used US postage stamps are worth 15 or 20 cents, the minimum amount required to catalog and stock the stamps. Unused will always be worth at least their face value. Special cancellations and different values are likely to be worth more. If it is truly an old stamp, it could range in value to a couple of dollars. If it is a really rare variety, it could be worth much more!