简化情况，不提cash out。打平的月份等于：
1）cost / monthly payment different
还是
2）cost / monthly interest different
上面两个都没计算延长的payment月份。
如果新loan一直付完，总收益 = old loan total interest - old loan paid interest - new loan total interest - cost。
这个直接 ：）
payment difference 比 intrest difference 要少很多。（当利率降了0.75% 以后）
感觉interest difference比较合理。
简化情况，不提cash out。打平的月份等于：
1）cost / monthly payment different
还是
2）cost / monthly interest different
上面两个都没计算延长的payment月份。
如果新loan一直付完，总收益 = old loan total interest - old loan paid interest - new loan total interest - cost。
这个直接 ：）
payment difference 比 intrest difference 要少很多。（当利率降了0.75% 以后）
感觉interest difference比较合理。