v.1. To negotiate some lower price: I bargained the price down by $200. They want $30, but if you try, you can bargain it down to $20. If that offer is too expensive, bargain it down some more.
2. To negotiate with someone to pay a lower price: If I can't bargain him down to $20 dollars, I won't buy the coat. The savvy shopper bargained the clerk down.