I had a quick look on Parkers guide for you and it reckoned the following:
3.5 V6 300 GT 3d
2006/56
Average mileage: 10,000 miles
Original Price £28,927
Franchised Dealer £23,530
Independent Dealer £22,755
Private Good £21,755
Private Poor £19,980
Part Exchange £21,310
There is only the mileage to account for but even with another 12,000 it won't affect the price more than about £500-1000. I think you probably bought at about the market price for your car.
Either way, I'm with Digsy. You've got it now and you're going to love it