Bottom line: If Melo decides he's going to Boston, he can sign as a FA and NYK gets NOTHING. So, NYK would have NO LEVERAGE in this case. Here's their choice: let Melo walk, and get NOTHING; or, agree to participate in a S&T with Boston to take back some underwhelming assets and expiring contracts. Then, they at least get something.
MIN, on the other hand, has all the leverage in the world. 16 teams (at least) are interested in Love. Love has virtually no control. Not even his threats of refusing to re-sign is deterring some teams (eg, CLE, SAC). MIN decides where and when to trade him, and they have plenty of offers to choose from. Demand is high.
That's why Melo's price could be so low, and Love's price could be so high. It's all about leverage.