A spoke was torn completley from the back wheel on my bike. its an 8 speed . what would be the cheapest course of action for me?

Take it to the local bike shop. They will be able to examine all the damage and details which have not been given, and advise what course of action is needed.

the rim cant be repaired its got a hole in it ripped out about a third of an inch in diameter. I guess ill just have to search garage sales etc till I find an old bike to salvage.

See if there is a bike co-op in your area. If so, the co-op may have a good wheel which was taken off a bike on which other parts were not salvageable. Take the old wheel with you for comparison.

Check online for prices.

Consider buying a complete bike.

You haven't said what type of wheel it is, or whether it is important for the new wheel to be the same. Consider replacing one or both wheels with something different but compatible.

1. Craigslist or ebay (local pick up)
2. Velomine.com (Sun CR18 rear wheels starting at $89)