Alex, are you sure your coin is not precious?
It sounds like you have a British half-crown from 1644, from the reign of Charles I, and these were minted in silver. The catalogs report very high values for these coins - if yours is well preserved and doesn't look silvery, it sounds like either a modern reproduction or a contemporary token of some sort, or at least heavily tarnished. You can see by the second picture below that modern reproductions can be obviously modern or seriously counterfeit. These sell on sites like ebay.co.uk for a few tens of US dollars.
DO NOT CLEAN YOUR COIN. CLEANING RUINS VALUE.
The coin in our main picture is not a fake. It is the genuine article and it was sold on auction by M & H Coins in London for £2700, or around $4100 US dollars. It's a beautiful specimen. Not many coins exist in that state!
The catalogs report the following values:
CROWN (approx. 44 mm diameter)
worn: $350 US dollars approximate catalog value
average circulated: $2100
well preserved: $4500
HALF CROWN (approx. 35 mm diameter)
average circulated: $1300
well preserved: $3000
It is worth mentioning that these coins come in many varieties. The inscriptions can differ, the reverse differs a lot (from a coat of arms, to an inscription under three fleur-de-lis), but the values do not vary too much.
The exception is the 1644 half crown with *weapons* portrayed beneath the king on a *galloping* horse. In our third picture, you can see an issue dated 1642 with this obverse design. If you have this design with the 1644 date, it's worth thousands of dollars!
In our last picture (with the black background, also from M & H Coins), you can see a half-crown with a 'B' under the king. This denotes the 'Bristol' mint, which as mentioned doesn't change the value much. An inscription of 'ox', 'oxon' or a variety in that location denotes the Exeter mint, like on your coin, Alex!
The variations can be subtle, and if you believe you have a genuine specimen of one of these coins, it would be wise to bring it to a coin shop, where a professional numismatist can take an in-hand look at the coin, and hopefully determine its authenticity. If you would like to send pictures to CoinQuest for our best guess at value, please start an e-mail exchange with us using the Contact Us link.
Alternatively you can have your coin graded by one of the officially recognized grading services: PCGS, NGC, ANACS or ICG. Look them up on the Internet. Do not use other services.