I think that $300 to $400 is more likely depending on condition because the Model 28 is the "cheapo" version of the Model 27 which goes for maybe $600 to $800 (also depending on condition).
They are the same pistols, but the 28 is the "Patrolman", a duty pistol with a lesser finish (matt instead of gloss blue) and some other cost saving features as well.
Internally, it is identical to the 27 but not as "pretty".
I'd consider the 28 to be a good range shooter or car gun, but I don't see lots of collectable value in it.
That may sound harsh, but it's similar to the difference between a brand new Mustang Shelby GT 500 and a new Mustang V6.
Same basic car, but done differently, and I know which one that I personally would find more desireable.
I've been given the opportunity to pick up a Model 28 from an estate at $200, and it's in really nice condition. But I'm tending to think I'll pass on it at that price. No one else wants it at this point, and I'm going to offer them $125 to take it off their hands, and if I get it for that, it will become another house gun to be stashed somewhere for quick access.