There are 3 levels. Cheap, Cheaper, and Cheapest. You decide what you can afford, and I'll give the ideas.
Cala lily table centerpiece
Cala lilys are a "cheap" form of table centerpieces pieces you can get away with putting in 3 of them in a tall vase that you find from a dollar store. Needing only 3 flowers definitely cuts down on the expense. Beef them up with greenery which too is inexpensive. Or, if cala lilys end up being too expensive, do the same with ferns, like in this picture.
Collect lots of inexpensive picture frames also from a dollar store. Copy your bride and groom picture, blow it up if you have to. Use the picture as your cheap wedding table centerpiece. Sprinkle rose petals around the picture.
Fill up these and put them in the center of each table. Most people like candy and the different colors of the candy add to the ambiance!
Believe it or not, sometimes you can rent potted plants from a florist. Ask for a large tropical plant and wrap tissue paper around its container tied with a bow. A green ficus wrapped in black and white toile tissue paper around its container looks great.