The average price difference in cost from regular to premium is $0.20/gal.
(i.e. Regular - $3.49/gal
Mid - $3.59/gal
Premium - $3.69/gal )
The savings on fuel from putting regular, rather than premium, is $2.40.
Honestly, I've been putting in Premium since I owned a Turbo Supra. My Lexus got premium as well. When I got the Focus, I was used to filling up on premium. Through previous threads on this, I'm pretty sure the ECU can take advantage of the higher octane.