Pricing are influenced by a lot of factors.
Please bear in mind:

Availability - is the flower in season, at the beginning of its available season, or at the end of its available season? (At the beginning and end of the season, flowers become more expensive as well).

Special days - Before and during Valentines day, Mothers' day, secretaries day and Christmas time, as well as a few other important days, ie. spring day.

Red and White flowers especially become more expensive at these times.

Weather - if there is for instance a few colder, rainy days during summer, the available amounts of some flowers suddenly become less than usual. Roses and lilies may also take longer to open from a tight bud to a beautiful open flower.

Flowers are fresh produce and because of this, we as florists are dependent on the prices paid at the market.

On a lighter side; somone once said: "Flowers are not grown in factories"