Cut flower gardening does not have to be expensive. The biggest costs are typically soil amendments, seeds/tubers, and support supplies. Here is how to minimize each.
Annual flowers grown from seed are dramatically cheaper than perennial plants or tubers. A single packet of zinnia seed (a few dollars) can produce 50+ plants. Cosmos, sunflowers, marigolds, nigella, and larkspur are similarly cheap from seed. Save dahlia tubers and divide them each year to expand your collection for free.
Compost is the best soil amendment and can be made for free. Many municipalities offer free compost from yard waste programs. Leaves and grass clippings make excellent mulch. Coffee grounds from local cafes add nitrogen.
Make plant stakes from fallen branches. Repurpose containers, buckets, and mason jars as vases. Use strips of old t-shirts as plant ties. Concrete reinforcing mesh is cheaper than commercial flower netting and works just as well.
Many cut flowers produce seeds you can save. Zinnias, cosmos, sunflowers, nigella, and marigolds all come true from saved seed. Let a few blooms go to seed at the end of the season and store the dried seed in a cool, dry place for next year.