SNT should ultimately be related to the cost of storage on the Network as you say.
However, for NRS, I’d argue this is one of the few areas where it would be advantageous for the cost to be decoupled from the storage size.
The purpose of an NRS name is in making a XOR address human readable/pronounceable, with a name that’s selected by me; that’s where the value is.
So it would ideally have a price significant higher than the raw storage cost of the string, but not unattainably high. So perhaps a dollar or two rather than $300.