One of my favorite centerpieces is to take a tall vase, some rocks, and a single stem of orchids to make a simple, chic statement. They are beautiful, modern, and best of all, cost under $10!
You can buy any size clear vase and bag of rocks at your local dollar store. Local florists can provide orchid stems in a variety of colors - we have a local orchid shop where I love to get mine for $2 a stem. Place the stem in the vase, and anchor the bottom with rocks. Trim off any buds that may go past the top of the vase. Slowly fill the vase with water to the rim, place on your table, and Voila! A simple, chic centerpiece!
You can experiment with different colored orchids and rocks. As an added touch, you can use a stem of orchids and cut individual flowers off, placing loose ones around the table for a more decorative effect.