For $100 more you get a dock, controller adapters, TV out, detachable controllers for multiplayer along with a kickstand for tabletop, rumble, IR, and a bigger screen. It's already a better value and it has seen bundles and price drops that will continue and get better around the holidays.
I think for now the OG Switch needs to be Nintendo's premium model so its gonna stay $300. My guess is that Switch Pro will actually just be the revised OG Switch (bigger screen/no bevel, faster, improved dock, similar size to make joy cons compatible) and will take the place of the existing Switch so that Nintendo isn't selling 3 different versions long term.