For a track-only car? I second the Miata/S2000 posts... Also C5Zs can be had for way under $20K... and you'd (probably, not sure of your track experience) be faster than either the 370Z/M3.
"Its only after we’ve lost everything that we’re free to do anything"
Current: 2007 Corvette Z06 (DD/HPDE)
Past: 2012 MazdaSpeed 3, 2007 BMW 335i E92 - 2001 Corvette Z06 - 2001 Camaro SS - 2007 CBR 600RR - 2003 CBR 600RR Racebike - 2001 CBR F4i