If you want to improve on the M42, M44 or any BMW head, you better know exactly what your doing. There is not a lot to be had, and its all labor so it = cubic dollars. I'd estimate about 3K dollars for a full on full performance titanium valved wonder. Tweeked cams, etc. Then add dyno time to exploit it, remember you have to find a dyno shop with a engine dyno set up for BMW engines. Its WAY cheaper to have a good engine builder just set your engine up for a DASC and go for it...
There is NO substitute for cubic inches of air + fuel.
Dave