Vertical spreads will trade between its minimum and maximum values - zero and the difference between the two strikes. In the case of a vertical call spread, the spread will trade closer to zero when the stock trades closer to or lower than the lower strike price. The spread will trade closer to maximum value when the stock trades closer to or higher than the higher strike price.

Remember, this maximum gain occurs at expiration. Before that, the spread will trade with a premium.

Starting from a stock price of 37.5, a price located directly between the two strikes, (using our example of the August 35 - 40 call spread) we can see the approximate value of the spread is roughly $2.50. This is because the August 35 calls and the August 40 calls are equidistant from the current stock price of $37.50. Being equidistant from the stock, both the August 35 and 40 calls will have almost the same amount of extrinsic value in them.

Thus, the extrinsic values of the two options cancel themselves out since you are long one call and short the other. This would leave each option value consisting of only intrinsic value. With the stock at $37.50, the value of the August 35 - 40 call spread will be $2.50. The August 35 calls will have $2.50 in intrinsic value while the August 40 calls will have $0 in intrinsic value. The difference gives you a spread with a value of $2.50.

A general rule of thumb is if the stock price is located evenly between the two strike prices, the vertical spread should be worth roughly half of the value of the distance between the two strikes. This will be true for vertical put spreads as well as call spreads. From this rule, we can roughly estimate the vertical spread's price per different stock prices.

For vertical call spreads, if the spread is worth roughly half of the difference between the two strikes with the stock price directly between the two strikes, then as the stock falls to lower strike and beyond, the spreads value will decrease and move closer to $0. Time left until expiration and volatility will dictate how close and how quickly it will approach $0. On the other side, as the stock climbs toward and above the upper strike, the spread's value will increase toward its maximum value described by the difference between the two strikes.

For vertical put spreads, as the stock price decreases toward the lower strike price, the spread will increase in value and approach its maximum value as defined by the difference between the two strikes. As the stock price increases toward the higher strike, the spread will decrease in value and will approach $0. Again, time until expiration and volatility will determine how quickly and how close the spread will approach $0.

Factors that Affect Spread Pricing
The determination of pricing as described above works in most cases. Be aware that it assumes that the implied volatility in both the 35 and 40 calls is the same. Most often, these two options will have a slightly different implied volatility.

This intra-month difference in implied volatility values through different strikes is known as a vertical volatility skew. The reason the markets run volatility skews is to make sure that out of the money options have enough premium in them to justify the individual option's risk/reward scenario.

Whatever factors affect the vertical spread, they are contingent on where the stock is in relation to the spread. Changes in implied volatility affect the price of a spread as stated above but the position of the stock in relation to the strikes of the spread is a key determinate of price.