I am getting ready to have a half cage added to my R56. I would second the recommendation for a custom cage. For a daily driver it is important that your head won't come near the bar in the event of an accident. IMHO this can be best handled with a custom cage. I got a price locally of around $800-$1000 for this. I think with any of the pre made cages you will be near or at this with installation anyway.
I am dropping my car off this weekend to have it put in. I will post some pics when it is finished.
Chase: check out Orbit 1 Racing at Motor Sports Ranch. They are a good custom cage builder that could do the welding for a pretty good price. Not sure where you are located (I live in Dallas) but it might be a good bet for ya. There site is:
http://orbit1racing.com/