Most I've ever spent to date, is $39K on the F150. It was 3 years old with 35K miles on it, completely optioned out as the highest trim package Ford sold at the time (the Platinum). The original sticker price was around $65K.
The Jeep Wrangler Rubicon a close second at $37K. It was also 3 years old but only had 15K miles on it.