If you're planning to create your own centerpieces, bouquets, bouts, favors and other wedding details it's a good idea to do a dry run first. A dry run will give you an idea of how much each item will cost and how long it will take for you to complete one item, then you can multiply the cost and time needed by the number you need to make.
You have to decide if you really have the time to create all of these items or if it's worth the expense of hiring a vendor or buying the products. Remember that time is money and you may be saving on the cost of each item but you may also be spending 10 hours a day for a few days putting them together.
And when it comes to flowers, remember that they need to be watered and should be refrigerated. They will start to wilt in a few days so you can't make them that far in advance. When you do a dry run on anything flower related, make sure to note how long they look fresh - 1 day, 2, 3, etc...