First, it is highly unlikely your coin was struck in 1780, the Maria Theresa thaler has been struck in large quantities from 1780 all the way to the present day. If you are really interested, there are ways to determine by the die if yours is an older strike or a newer one. But generally, these coins sell for silver scrap, or perhaps a bit more. You should easily be able to sell it for $20-25 depending on the price of silver.
There are a few things to realize about a 1780 Maria Theresa Thaler: Your coin was NOT minted in 1780, the Austrian mint has produced Maria Theresa Thalers fixed at Maria Theresa's death in 1780, most likely your coin was made in the 20th or 21st century. That doesn't mean your coin is worthless it just means that your coin isn't several hundred years old. Since they are restrikes, they don't have much of a premium over silver value, today they run around $25-30 depending on silver price and the condition of the coin.
Most Maria Theresa Thalers are modern restrikes and generally trade for close to silver melt, about $20-21 at present. Don't let the date fool you, nearly all the coins were minted well after 1780 and indeed continue to this present day, still with the date of 1780.
It's not really a silver dollar. What you probably have is a Maria Theresa taler, a bullion coin that's been issued by many countries almost continuously for over 2 centuries. All of them are dated 1780 regardless of when they were actually minted. Unless it's an original coin (very unlikely) its value is roughly the same as the current price of 1 oz. of silver. Silver prices change daily so it's best to look at a precious-metals site or the financial pages of a newspaper for current values.
Anna Maria Porter was born in 1780.
I assume you're referring to a Maria Teresa thaler, probably dated 1780.This is actually a bullion coin that's been struck in a number of countries, with the same date and design ever since then. It's sold for its silver value, not for spending. It sells for a bit more than the price of 1 oz of silver. That number changes a lot so you'll need to look in the business section of a newspaper or Google silver AND "spot price" to get an idea of its current value.
More than 800 Million have been struck to date, all dated 1780 The thaler became the unofficial currency in some areas of Africa and Asia, and is still in use today as a "trade silver dollar" in many Arabian bazaars.
Maria Theresia of Tuscany died on an unknown date in 1780.
José Maria de Zalvidea was born in 1780.
Maria Theresa
Maria Theresa
Duchess Maria Antonia of Bavaria died on 1780-04-23.
$18 is the value of a 1780 x countermark coin 20 piastres.