The market value of that home may have doubled to $800,000. But because the down payment locked in a pre-determined price, the buyer pays $400,000. Now, in an alternate scenario, say the zoning approval doesn’t come through until year four. This is one year past the expiration of this option. Now the home buyer must pay the market price because the contract has expired. In either case, the developer keeps the original $20,000 collected.
Purchasing a call option is essentially betting that the price of the share of security (like a stock or index) will go up over the course of a predetermined amount of time. For instance, if you buy a call option for Alphabet (GOOG) at, say, $1,500 and are feeling bullish about the stock, you are predicting that the share price for Alphabet will increase. 
Arbitrage arguments:  When the commodity has plentiful supply then the prices can be very well dictated or influenced by Arbitrage arguments. Arbitrage is basically buying in one market and simultaneously selling in another, profiting from a temporary difference. This is considered riskless profit for the investor/trader. For example, if the price of gold in delhi is INR 30,000 per 10 grams and in Mumbai gold price is INR 35,000 then arbitrageur will purchase gold in Delhi and sell in Mumbai
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Whether the economy is hot or not, an investor can make money trading commodity options, regardless of the condition of the market. Briefly, a commodity option allows its owner to either sell or buy a commodity like corn or wheat at a future date. You will buy a so-called “put option” if you think the price of the commodity will go down and a “call option” if you think the price will rise. And never will you have to take possession of the commodity itself.

Time Value: All options contracts have an expiration date, after which they become worthless. The more time that an option has before its expiration date, the more time there is available for the option to come into profit, so its premium will have additional time value. The less time that an option has until its expiration date, the less time there is available for the option to come into profit, so its premium will have either lower additional time value or no additional time value.
The price at which you agree to buy the underlying security via the option is called the "strike price," and the fee you pay for buying that option contract is called the "premium." When determining the strike price, you are betting that the asset (typically a stock) will go up or down in price. The price you are paying for that bet is the premium, which is a percentage of the value of that asset.