That car looks great - I'm assuming that the RHD conversion was well done.
Much of the value is buyer dependant - if the buyer:
1] wanted a great colour combo that looked great
2] wanted big-block mustang
3] wanted a shaker car
4] didn't care about originality (shaker, rear sway bar and an oil cooler on a S code for examples??)
5] had a lot of money burning a hole in his pocket
then there's a seat for every wallet.
To be fair, this was really 428CJ or SCJ pricing for a top quality restoration, nice colour, OEM panels, 4-speed, original parts (valve covers, distributor, air-cleaner, oil cooler setup, Carter X fuel pump, snorkel S-Tube and heat shield for example) would justify that price. The seller got a GREAT price, the buyer got a lovely car.
Best of luck to them both....