I have a Australian delivered 2002 Cobra, I installed a 2003 S/C Cobra drivetrain from a wreck in the US.
The engine/gearbox etc has about 65+K on it now, it cost about $20K to swap out everything out - it's also had the seats swapped, gauge cluster, exhaust, fuel system, trunk & spoiler/hood/bumpers/scoops, wheels and Brembo brakes from a Cobra R etc. It's as close as I could get to a Terminator Cobra in Australia without going through everything your seller did.
I upgraded the Eaton to a Whipple and it's a monster 425rwkw/971NM - they're cool cars and 7/800hp with the right add-ons is reliable due to the forged internals and Manley Rods as standard items.
In the US the value of the 03/04 cars is going up - they're knocking on the door of $30KUSD for a great, low-mileage example now, but the market here in Oz if different and people think of the Tickford Cobra not the Terminator Cobra.
I'd value my car at around $40K on a good day and a Convertible slightly less - although the fact that your car is an original 03 would negate that.
Just my opinion...