Keep it simple. Flower boxes are easy and elegant, cheaper than vases, and hold more flowers. Plus, they are low to the table and won't interrupt table conversation among your guests. Ever have to do that stoop or stretch routine to talk to a wedding guest sitting across a table from you? It's annoying.
Here's the flower box plan:
Buying garland will give you more bang for your buck, one garland will make two centerpieces. Wooden flower boxes can also be made for cheap!
Need more centerpiece ideas? Check out Centerpiece Cents where all centerpieces cost five dollars or less!