This jacket was on sale at a good price. - A good price is really a number.
They live at 22 River Street. - Once again, a number.
They live on River Street. - There's no number here.
The movie starts at 10:00.
The movies starts at around 10:00.
Prices start at $600.00.
Prices start at around $600.00.
You can reach me at 999.888.7777. - Once again, a number is a location.
Use "at" for locations.
He's at the post office.
Don't point that thing at anyone. It's dangerous. - Anyone is a target. A target is a location.
Wherever anything starts, that place is a location. It starts at one and then continues until six. It ends at six.
Numbers are locations. Numbers are prices. Prices are locations.