I saw a 2004 330ci slick top 6MT with 27,000 miles in mint condition sell for no more than $18,000 (almost bought it myself). Therefore, since your car is a convertible with 36,000 more miles, I would assume you’d get less, even as a ZHP. As a ZHP, I’m guessing you’d get $15,000, give or take, for a nice specimen. I’m no expert, but I’ve been tracking such cars for years.