Knowledge
You are paying for knowledge. I know, I know, everyone is a florist, photographer, or event planner these days. But the people in these industries have devoted their lives to this art.  They have knowledge beyond a single DIY video or planning one event.  I personally have an Associate’s degree in floriculture merchandising and a Bachelor’s degree in business administration with a focus on marketing, plus I bring 22 years of experience in the industry.
Supply + Demand
Styles are not changing as quickly as they used to due to the internet, so blush and cream flowers are in high demand. Seasonal availability is also a major factor. Additionally, natural disasters in areas of the world can impact availability (for example, flooding on rose farms has caused a shortage of roses this spring).