Oxford Guide to English Grammar; John Eastwood; Oxford University Press 1994-09
We can also sometimes put a complement in front position.
They enjoyed the holiday. Best of all was the constant sunshine.
The scheme has many good points. An advantage is the low cost.
Here the subject (the low cost) is the important information and comes at the end.
How about viewing "an advantage" as the subject while viewing "the low cost" as the subject-complement, and there being no inversion?
How about viewing "best of all" as the subject while viewing "the constant sunshine" as the subject-complement, and there being no inversion?