Tom loves Ernestine and Suzanna knows. That's why she's particularly cruel towards her and her children. Tom doesn't love his wife, he hasn't even been there to see the birth of his children. I also think the price to capture Rose alive is because that's his daughter. Even in explaining the slaves for the "Wanted" poster, when he gets to her name he slows down and has this almost worried tone in his voice. Tom is in a situation where he wants what he can't have the way he wants to have it. Ernestine is more than just a slave to him.