no prevailing rate is not rack rate as far as I know
but its also a bit of a grey area as to what rate one needs to pay to get a suite upgrade.
I think it depends on the property .. at least in my experience it does.

you can turn sommersalts in GHHK and usually not get a suite
you can pay the BN992 (2 nights for $359 inc $99 in f and b)

and get the suite with a certificate in Adelaide

and everything in between
go figure.

Id call and ask the res agents and get a name and make sure the upgrade is requested on the reservation confirmation