Two PSUs providing the very same real amount of wattage might represents different costs in the long run.
Say one of them has 500W nominated but provides actually 400W.
The other has 600W nominated, providing in reality the same 400W.
Though the 600W one, which has a 66% rate of actual usage will be cheaper than the 500W one (80% of usage), if power bills are taken into account, the latter will prove cheaper, for it'l provide the same 400W for your machine while using 500, instead of 600.
Gotta take all of it into account.