As mentioned, $300 is very unrealistic to fly most of the way across the country at short notice.

Best I can find is $416:

Depart – Thu Aug 5 2010
Delta
Flight 2235 Departs: 5:25p
Wayne County (DTW) Arrives: 6:24p
Minneapolis St Paul (MSP)
Coach | Fare code: UE10A0SJ | Airbus A319 (Narrow-body Jet) | 1h 59m | 60% on time | 526 miles | 1 seats remain
Layover in Minneapolis, MN (MSP) for 1h 01m
Delta
Flight 2705 Departs: 7:25p
Minneapolis St Paul (MSP) Arrives: 9:29p
San Francisco (SFO)
Coach | Fare code: UE10A0SJ | Boeing 737-800 (winglets) (Narrow-body Jet) | 4h 04m | 30% on time | 1585 miles | 1 seats remain

Return – Mon Aug 9 2010
Delta
Flight 2412 Departs: 1:30a
San Francisco (SFO) Arrives: 7:35a
Memphis (MEM)
Coach | Fare code: UE10A0SJ | Airbus A320-100/200 (Narrow-body Jet) | 4h 05m | 80% on time | 1802 miles | 1 seats remain
Layover in Memphis, TN (MEM) for 0h 55m
Delta
Flight 2308 Departs: 8:30a
Memphis (MEM) Arrives: 11:25a
Wayne County (DTW)
Coach | Fare code: UE10A0SJ | Airbus A320-100/200 (Narrow-body Jet) | 1h 55m | 70% on time | 610 miles | 1 seats remain


On a side note, a friend who works in the business tells me that the business model of airlines is such that you should never expect a sub-$400 fare for an 1800 mile trip.