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.
2004 BMW 330i ZHP (52k miles), Jet Black with black leather, MFactory LSD (3.38) with Z4 cover, 18"x8.5" ET38 APEX Arc-8's (Anthracite) with 245/40 Michelin PSS', Koni Sports with factory springs and front and rear reinforcement plates, Shark Injector, Corsa TSE3 cat-back exhaust, TMS under-driven pulleys (water & steering), CDV delete, TMS CF strut brace, K&N CAI, GAS DISA rebuild, TMS pedals, Wheelskins steering wheel cover, roller tray center console, black-out grilles, and WeatherTech mats