Cheap wedding centrepieces (if done correctly) can finish your table decorations perfectly.
However they can also ruin the overall look if done incorrectly and they are normally the most visual reference point on your guests table setting.
When considering what type of inexpensive centrepieces you should go for there are (as always) a number of things you should consider:
Talk to your venue - always talk to them before you buy anything, (it will only take a few minutes and it may prevent you wasting money buying something you're not allowed to use). Who knows, they may even be able to help you save money. Our venue had beautiful (very large) candelabras which they allowed us to use free of charge. If your venue can help you out like ours did it will obviously save you from having to buy or rent them yourself!
That way you will get the true cost of your cheap wedding centrepieces (and don’t forget to allow adequate time to prepare them and set them up in the venue, if you are doing them yourself). A cheaper alternative to flowers is to use candelabras and decorate them with ivy and a few well placed individual flowers and candles. Another alternative is to use balloon centrepieces (much cheaper and is relatively easy to do yourself, so you can save the cost of a florist) or by using balloons you could just reduce the number of flowers you need to buy.
As with most elements of your wedding planning, it's very easy to get carried away. So before you commit to anything, step back consider it rationally (is this something you have to have or are you just getting caught up in the romance of it all?). Does it fit into your budget? If not don't be tempted to put it in, you don't need it.
Consider the cost. If your cheap wedding centrepieces prove too expensive by the time you multiply the cost for all the guest tables, then simplify them. This will serve two purposes: (at the risk of repeating myself, less can be more) it will potentially make them more elegant and more importantly it will save you money.