What is meant by 废钱？
Your reference is very likely comes from this Japanese Chinese teaching book 1, and the character "钱" actually is a typo of "铁", this is a very common mistake when people type Chinese in Wubi Input Method (or 五笔字型输入法". 

废铁 is scrap metal (for recycling), it perfectly fits the surroundings (后院 backyard), put it in the context this sentence's meaning: 
If you can trade this this not valuable thing (废铁) to a way much more valuable thing (汽车) for me, I'll be not only happy to agree but also surprised.

You can see in the linked book, the conversation is the girl (maybe a producer) asks Mr. Wang, will he agree to let them adapt his Internet novel to a TV show, so Mr. Wang made a humble and humorous metaphor. (The context is Mr. Wang wrote more than 30 novels, only two of them got published)