(CNN) - Gas prices decreased from their all-time high by about half a penny over the past two weeks, to $2.30 per gallon of self-serve regular, a national survey said Sunday.

With ample supplies of crude oil and gas, prices could drop further, said Trilby Lundberg, publisher of the Lundberg Survey. "If crude oil prices stay put or soften from here, then gas prices will follow suit," she predicted.

Lundberg said the lowest average price in the July 22 survey was in Fargo, N.D., where a gallon of self-serve regular cost $2.11; the highest was San Diego, Calif., where a gallon cost $2.58.

Here are some prices in other cities:

Atlanta: $2.21

Boise, Idaho: $2.33

Hartford, Conn. : $2.45

Houston: $2.23

Philadelphia: $2.42

Portland, Ore. : $2.30

St. Louis: $2.14