Use inexpensive blooms like carnations or baby's breath, or use succulents or potted plants. Skip big table arrangements and orchids. Get in-season blooms, and focus on fun colors of common blooms instead of expensive flowers.
For budget- and eco-smart vases, hollow out fruits and vegetables for inexpensive vase alternatives. Pumpkins, watermelons, squash — they all work great!
Consider doing a work trade with a local florist or flower farmer.