Black and white seems to be a very popular theme. It is relatively easy to do too. White or black platter, a few candles, voila. Done. Here's a few, all under $10 each and really easy to do.
This first one is simply ribbon around some white candles of varying heights, on a platter, which can easily be picked up for $2 - $5, and the candles for $2 each. The ribbon is $2 for a roll, but you wont use the whole roll on one centrepiece.
For something different you could add some pebbles, which are $2 a box, but a box will do quite a few platters.
Or you could just leave out the platter and do it straight on the table.
Some other ideas are layer black and white pebbles or sand in a vase
Have a few black and white flowers in a vase
Have a glass bowl with black pebbles on the bottom and white floating flowers on the top or vice versa
Black and white is classy and elegant. It is very simple, yet looks absolutely stunning.