书上计算NPV的案例：
Example: NPV analysis for Douglas Manor Apartments (DMA):
1. purchase price=$525,000(rounded);
2. Net operating income(NOI) in year1= $64,000;
3. Assumed to be held for 4 years, during which NOI growth rate=5%
4. Improvement cost =$451,000(improvement are assumed to make up 85.9% of the $525,000
purchase price); 27.5 year, straight line cost recovery; Tax depreciation= $16,399,
这里improvement cost这个概念我不是很明白。是总折旧么