Oh, wow. What a gorgeous coin! The British minted this coin for India only in 1918 out of 0.235 troy ounces of gold, the same weight as the British sovereign gold coins. That's where to start the value calculations, with the gold content. Once you have the value due to gold, add more value due to collector appeal.
The value due to gold is 0.235 times the current value of one troy ounce of gold. According to kitco.com, the value of gold today is $1241.53 US dollars per troy ounce. Tomorrow the value will be different; it changes every day, so be sure to look it up. Multiplying 0.2355 x 1241.53 gives $292.38 US dollars gold value.
To the gold value now add a collector premium due to demand from numismatists (coin collectors). Here are some typical premiums:
worn or damaged: add $400 US dollars
average circulated: add $1000
well preserved: add $1800
fully uncirculated: add $2200
It is very important to protect a coin like this from wear and the effects of external elements like humidity, chemicals, and fingerprints. A good way to do this is to have the coin slabbed, that is, mounted in plastic, by a third party grading service. Use only PCGS, NGC, ICG, or ANACS services. Do not use others. Look them up on the Internet.