Scott #1298 dark brown on white?
Mint a copy can be purchased for 50 cents. Used it is 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.
There are some varieties with higher values, so consult a postage stamp catalog, usually available at your local library for exact identification and catalog values.
Six cents.
I believe you are referring to a 23 cent stamp. That is Scott Number 2181. It has a minimal value and can be purchased for about 20 cents.
That is Scott number US 932 It has a value of 40 cents mint and 15 cents used.
The lowest value US stamp was the 1/2 cent. A couple different versions were issued.
22 cents.
The US ten cent Christmas stamps have minimal value. But if it isn't US, what country issued it, whether they are used or mint, and their condition all affect value. Consult a stamp catalog for the country in question and identify the stamp. It will also assist you in finding the value.
25 cents retail
The 1945 Lincoln cent is a very common coin. Most are only 3 to 10 cents.
The US has issued many dozens of red 3 cent stamps. It will take a bit of work to identify which stamp you are referring to. I would recommend spending some time looking at a US Postage Stamp Catalog to identify the specific stamp. The bonus is that most catalogs will give you an idea of value.
15 cents.
It has a face value of one cent. It is not very rare and you can by a copy for the minimum stamp fee of about 20 cents.
That is Scott number US 2418 It has a value of 1.30 mint and minimal 15 cent used.