2006 Mustang V6 Coupe Standard - $19,115 MSRP (4.0L V6, 210 HP / 240 TQ)
2006 Mustang GT Coupe Deluxe - $25,140 MSRP (4.6L V8, 300 HP / 320 TQ)
2006 G35 Coupe 6MT - $33,800 MSRP (3.5L V6, 298 HP / 260 TQ)
2006 350Z Coupe - $27,650 MSRP (3.5L V6, 300 HP / 260 TQ)
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2011 Mustang V6 Coupe - $22,145 MSRP (3.7L V6, 305 HP / 280 TQ)
2011 Mustang GT Coupe - $29,645 MSRP (5.0L V8, 412 HP / 390 TQ)
2010 G37 Coupe Base - $36,050 MSRP (3.7L V6, 330 HP / 270 TQ)
2010 370Z Coupe Base - $30,410 MSRP (3.7L V6, 332 HP / 270 TQ)
In conclusion, in all cases the Mustang GT is both cheaper and more powerful than the G37 or 370Z. In the most recent models, the Mustang V6 comes close to the G37 / 370Z's HP and TQ numbers while being $8-14 THOUSAND cheaper based on MSRP. I'm not trying to imply that either are bad cars - they are't - but you have to realize that they're more luxury oriented and thus carry a higher price premium while the Mustang GT is more of a true muscle / sports car.
It's been said dozens of times, and I'll say it again. The latest Mustang offerings are very hard to beat if you want to go fast for cheap.