Waistcoasts are suit parts and need to be cut from the same fabric as pants and jacket. If it's truly that much of a barrier, you can save a little money by getting suit and waistcoat, with a contrasting red tie. But then you need a really good shirt as well. It would end up saving you money and you wouldn't look out of place indoors at the party, but when the situation gets more formal as in the ceremony or when you're anywhere else really, you will look off. The money you save isn't worth having a costume, that is not complete and you can't compliment it with a jacket later, because as I said, a suit needs to be cut from the same roll of fabric at the same time so they wear at the same rate. Just put in the extra hours to thrift store hunting and working a few extra hours.
First you want to go see the really expensive suits in the mall so you can get an idea of what looks nice, because you can get some real crap in thrift stores.
