My wife's 2011 128i Cabrio with 6MT, M-sport, leather/heated/power seats, and a few other small options stickered at about $40K, nearly $5K less than a 135i Cabrio. I put a BMW Performance Exhaust on it and we both really like the sound. Speaking of which, the 2011 128i comes with HD Radio, and the sound system is pretty good. The optional one is about $900 extra, and in a convertible I'm not sure it would have been worth it. Also didn't go for the HID headlights, as this was another $900 option - the halogens are fine for this car. I am adding some wider 18" wheels and non-runflat tires for better ride/handling, which will cost about what I'd have spent on the sound and headlight upgrades.
2013 F30 335i M-Sport 8AT/MPE/MPPK, 2011 128i M-sport Cabrio 6MT/PE