Can someone help to justify this? broad example $300 p/w or...

Can someone help to justify this?

broad example
$300 p/w or $15,600 p/a x 8% = $1,248
$450 p/w or $23,400 p/a x 8% = $1,872

Does the $450 p/w rental really get the extra care to justify the extra $624? Or does it really just get the exact same treatment as the $300p/w rental? What would 'extra care' involve?
If not, then why is there not more use of a flat fee per residence? Why is the usage of a standard percentage rate so prevalent?