Hi Paula -- Your half sovereign derives its value from its gold content. A half sovereign contains 0.1177 troy ounces of pure gold, and a full sovereign contains 0.2354 ounces.
Given the current value of gold, approximately $1340 US dollars per troy ounce at this writing, the value of Paula's coin is 0.1177 x $1340 = $158. That is the base value of your coin. Tomorrow it will be different, because the value of gold changes every day. Look it up at kitco.com.
If Paula's half sovereign is in really good shape, collector demand will push the value upward by about $50. If your coin is cleaned, scratched, spotted or otherwise damaged, down it slides to base value.
Paula's coin is a half sovereign, but you might have a full sovereign. The designs are the same, but here's the difference:
HALF SOVEREIGN: 19 mm diameter, 0.1177 ounces of gold
SOVEREIGN: 22 mm diameter, 0.2354 ounces of gold
To get a rough value of a sovereign, multiply the value of gold (from, e.g., Kitco.com) by 0.2354 then, if the coin is in good shape, add $50 or so. The $50 is called collector premium and represents the amount a collector is willing to pay over and above gold value to add a coin to his or her collection.
Just about all dates of sovereigns and half sovereigns obey the $50 premium rule, even going as far back as Queen Victoria. If you do a search on 'sovereign' (box at upper right), you will see separate pages for different date coins.
Now there is always a chance that your Edward sovereign or half sovereign has a mint mark. Look closely (I need a magnifier to see it) on the reverse side under the dragon slayer. Our picture shows and M mint mark on a coin dated 1911.
Here are some dates and mint marks that carry extra value. The dollar figures after the date and mint mark are catalog values for coins in well preserved condition.
C = Canada
P = Perth, Australia
M = Melbourne, Australia