Nice coin, Jen. The double headed eagle gives away the design as being from Austria, Russia, or Germany. You have a Maria Theresa Thaler from Austria: a big, bold, beautiful coin.
Because these have been issued over and over again since 1780 by dozens of different mints around the world, they are worth only their bullion value. Each coin contains 0.7516 troy ounces of silver, so their bullion value is 0.7516 times the current price of silver. If silver is trading at, say, $21.20 US dollars per troy ounce, the bullion value is 0.7516 x 16 = $15.95. Be sure to use the current value of silver. Look it up at kitco.com. It changes every day.
If you have a Maria Theresa 1780SF restrike that is in superb shape, you might be able to get somewhat better pricing, as follows:
worn: add $0 to silver bullion value
average circulated: add $0
well preserved: add $2
fully uncirculated: add $10
The original Theresa Thalers were issued in the Austrian State of Burgau in 1780. The originals look essentially identical to your restruck coin except for a few minor variations. But, starting in 1781, the exact same coin, even with the exact same date, was struck by Austrian and other mints and this continues to this day. The coins are used as 'Trade Coinage' in locations where local coins are not available. One of my catalogs estimates that 800 million of these coins have been struck.
You have a nice coin. But it is not very valuable.
There's a great description of the differences between an original coin and a restrike at 1messydesk.com.