Why use percentages instead of real numbers?

[Ken Virzi]

When crunching the numbers all I seem to see people using are percentages. Obviously some things are, such as mamagement. But most costs are t based on percentages. For example, a roof costs the same for a building that rents for $900 vs one that rents for $1400. With a capex percent based line the $900 rental would have less then the $1400 rental but the actual cost would be the same. 

Add replacing or fixing a toilet, lawn care, sometimes even insurance and it seems to be most costs would be a higher percent the lower the rental amount. 

If we used actual numbers when looking at the numbers would anyone buy a place with less than $1,000 under one roof?