They say it can't be done. Flowers are often a very inportant and very expensive part of a wedding ceremony/reception decor. There are ways, however, to save in this department.
If you are low on funds after paying the DJ and the hall, before you even talk with your florist, pick a color scheme rather than a particular flower. Then you can ask your florist to choose whatever is available in your palette at the best prices out there. If roses are a must have, you can always have more roses on the head table, than on the guest tables, for example, and supplement another red flower there insead. You should be keeping in mind what is in season.
While you probably won't be able to save money by creating your own arrangements because of time, if a friend is willing, you can save a bundle by ordering in bulk from a wholesale or farmer’s market. Also, some Votec classes may be willing to make a classroom assigment out of it!