Hopefully you can see - these are actually decorations to our first formal this year and they were reaaally inexpensive. the vases actually were bought in bulk and we'll be reusing them because they cost just the same amount (if not less) then renting them.
It's easy to go to a bulk barn and find candies that are your organization's colours and make 'goody bags' for everyone - balloons as well.
We had ours at the penthouse in nice hotel, who provided the rest of the decorations, the microphone, and the stand for speeches.
Our semi was actually done at a fancy italian restaurant - we just made reservations and had it as if it were a nice 'party' dinner out that way we could choose and pay for the food we wanted. No crazy decorations at all, but still easy to dress up nicely. Because it was our first semi, we also only invited our Sweetheart for our dates. It was a lot more intimate and nice, and a lot of people told us afterwards it was a nice idea.