You are lucky, there are new and used many cars for under 30k.
New Cars:
2009 Honda Accord EX-L V6 Coupe (FWD, 0-60: 5.7)
2008 Nissan 350z ( RWD, 0-60: 4.9)
2010 Ford Mustang GT (RWD 0-60: 4.9)
2009 Hyundai Genesis Coupe (RWD 0-60: 5.4)
2009 Mazda RX-8 (RWD 0-60 5.6 sec)
2009 Dodge Challenger R/T (If you Haggle) (RWD 0-60 5.5 sec)
2009 Chevy Camaro SS ( If you haggle a lot) (RWD 0-60 4.5 Sec)
2009 Nissan Altima Coupe 3.5 S (FWD 0-60 5.8 sec)
2009 Nissan 370z (RWD 0-60 4.7 sec)
2009 Dodge Challenger SE (RWD 0-60 7.7 sec)
2009 Mitsubishi Ralliart (AWD 0-60 5.5 sec)
Used Cars For Under 30k:
2005-2007 Infiniti G35 Coupe w/ Navigation & Sport Package (RWD 0-60: 5.4 sec)
2008-2009 Infiniti G37 Coupe Base Model ( RWD 0-60 4.9 sec)
2006-2008 Ford Mustang GT500 ( RWD 0-60 4.9 sec)
2006-2008 Nissan 350z w/ Nav & Sport (RWD 0-60:4.9)
2007-2008 Acura TL Type S With Nav (FWD 0-60: 5.5)
2002-2006 BMW M3 Coupe Auto (RWD 0-60: 4.6 sec)
2006-2008 Lexus IS350 (RWD 0-60: 5.5 sec)
2004-2008 Honda S2000 9000 RPM Redline (RWD 0-60: 6 sec)
2007-2008 Saturn Sky Turbo (RWD 0-60 5.5 sec)
2002-2004 Maserati Coupe /Convertible (RWD 0-60 5.3 sec)
2005 Lotus Elise (RWD 0-60 ?)
2006-2008 Pontiac GXP (RWD 0-60 5.4 sec)
2005-2009 Ford Mustang GT (RWD 0-60: 5.5 sec)
2004-2008 Cadillac XLR (RWD 0-60 5.4 sec)
2005-2006 Chevrolet Corvette (RWD 0-60: 4.6 sec)
2004-2008 Mazda RX-8 (RWD 0-60 5.8 sec)
1992-2002 Dodge Viper (RWD 0-60 4.7 sec)
1999-2004 Porsche 911 Carrera (RWD 0-60 4.8 sec)
2004-2005 Pontiac GTO 6.0 (RWD 0-60:5.5 sec)
2008 Pontiac G8 GT (RWD 0-60 4.9sec)
2005-2008 Audi RS4 (AWD 0-60 4.6 sec I think)
2000-2004 Ford Mustang Cobra (RWD 0-60 4.7 sec)
1990-1996 Nissan 300zx Twin Turbo for Under 10k (RWD 0-60 5.8 sec)
AND WAY MORE
You are better off with a Used Car because most of these you can get under 20k miles and most of them are selling below 20k. The rest you can use for customizing or college. If I were you I would go with an 2005-2007 Infiniti G35 Coupe because this car is blister fast (trust me) and they will come with a nav and sport package but just save another 7 grand and buy a G37 for 37 grand and you get an Infiniti G37 with 7 inch standard screen, 7 speed automatic, 3.7 litre V6 athat get 26 mpg with 330 hp and 270 torque,19 in rims, 14 in rotors with infiniti caliber 4 piston and 13.8 inch 2 piston infiniti caliber. Translate to 4.9 sec run with 107 feet stopping power. The exhaust note it just spectacular.
Used Cars For Under 20k:
2005-2007 Infiniti G35 Coupe w/ Navigation & Sport Package (RWD 0-60: 5.4 sec)
2006-2008 Nissan 350z w/ Sport Package (RWD 0-60:4.9)
2002-2006 BMW M3 Coupe Auto (RWD 0-60: 4.6 sec)
2004-2008 Honda S2000 9000 RPM Redline (RWD 0-60: 6 sec)
2007-2008 Saturn Sky (RWD 0-60 7.5 sec)
2006-2008 Pontiac Solstice (RWD 0-60 6.7 sec)
2005-2009 Ford Mustang GT (RWD 0-60: 5.5 sec)
2004-2008 Mazda RX-8 (RWD 0-60 5.8 sec)
2004-2005 Pontiac GTO 6.0 (RWD 0-60:5.5 sec)
2000-2004 Ford Mustang Cobra (RWD 0-60 4.7 sec)
2000-2004 Porsche 911 Carrera (RWD 0-60: 4.8 sec)
2004-2008 Acura TL (FWD 0-60: 6.8 sec)
2005-2007 Audi A4 2.0 T (FWD 0-60: 7 sec)
2005-2008 Chrysler 300C (RWD 0-60 7.7 Sec)
2002-2006 Jaguar X-Type (RWD 0-60 8 sec)
2000-2004 Jaguar S-Type (RWD 0-60 7.7 sec)
AND WAY MORE!