That's what I was going to say Shady. No one had a clue that the new 5.0 engine was on the horizon when your dad bought the 2010 GT. If it was popular knowledge then, not as many 2010s would have been sold.
Im pretty sure it was common knowledge in the mustang community that '11 would have a new motor way before the '10s hit the lots.
Look at all the shit were hearing about the '14 models already and its only 2011. Sure most is speculative but theres common sense that tells you to not buy a '13.
And id bet the sales figures for '10 GT's suck ass, and all likelyhood they only built half as many as compared to '09.