I like to use a formula like this for old British sovereigns and half sovereigns:
Retail value = Melt value + Collector premium
This page applies to half sovereigns with Victoria on the front and a crowned shield design on the back. In 1887 the queen's portrait changed, as shown in our secondary picture.
The melt value is what you would get if you used a blow torch to turn the coin into a pool of precious metal. There are 0.118 troy ounces of gold in a half sovereign, and with the current weak dollar, gold today sells for about $1150 US dollars per ounce. (This is the gold value at the time of this writing. Use web sites such as Kitco.com to find the current price of gold. It changes every day.) The melt value, then, for a half sovereign is 0.118 x 1150 = $138 US dollars.
Now add the collector premium to the melt value amount. The collector premium comes from the fact that collectors want to place your coin to their collections, and will pay a premium for that privilege. But collectors are finicky folks. If your coin has scratches, spots, scrapes, or has been cleaned, the collector premium goes to zero. Otherwise, for problem-free coins, here are tables of collector premiums:
HALF SOVEREIGNS 1838 TO 1880
Worn: melt value + $20
average circulated: melt value + $150
well preserved: melt value + $400
fully uncirculated: melt value + $800
HALF SOVEREIGNS 1887 TO 1893
Worn: melt value + $20
average circulated: melt value + $50
well preserved: melt value + $100
fully uncirculated: melt value + $250
There are a few special dates which really stand out in value. Half sovereigns dated 1862 are very rare and catalog around $7000 for a well preserved specimen. Other dates -- 1845 and 1850 -- are somewhat rare and catalog around $2000 in well preserved condition.
Finally, get out your magnifier and look under the shield on the reverse side. You may see a small annotation, sometimes S (as in the photo), sometimes M, and sometimes a number. The letters are mint marks and the numbers are die numbers. If you find one, seek out a knowledgeable collector or dealer to investigate your coin further. Die numbered coins from Great Britain are usually worth a little more than those without die numbers. Certain dated and certain mint marks, also, are worth more.