These old sovereigns derive their value from gold content. The designs are the same for half sovereigns and full sovereigns, but here's the difference:

HALF SOVEREIGN: 19 mm diameter, 0.1177 ounces of gold
SOVEREIGN: 22 mm diameter, 0.2354 ounces of gold

Given a current value of gold, let's say $1340 US dollars per troy ounce, the value of a half sovereign is 0.1177 x 1340 = $158. That is the base value (BV). BV for a sovereign would be 0.2354 x 1340 = $315. The value of gold changes every day. Look it up at kitco.com.

For coins in excellent condition, you can add a collector premium to the BV, like this:

SOVEREIGNS AND HALF SOVEREIGNS (no mint mark, see below):
worn: BV
average circulated: BV
well preserved: BV + $20
fully uncirculated: BV + $50

If your coin is cleaned, scratched, spotted or otherwise damaged, it is worth base value.

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. Other mint marks are C and P:

C = Canada
P = Perth, Australia
M = Melbourne, Australia

Some coins with mint marks carry extra value. The dollar figures after the dates and mint marks below are catalog values for coins in well preserved condition.

HALF SOVEREIGN:
1904P: $1400 well preserved
1906M: $1000
1907M: BV + $100
1908P: $1500
1909M: BV + $100
1909P: $600

SOVEREIGN:
1908C: $3000 well preserved