Depending on which estimate you believe, legal expenses (including direct and indirect) account for about 1-3% of overall US healthcare costs. Not nothing, but not in the range of what we're spending on, say, end-of-life hospital care (10-20%).
Okay, fair enough. I remember reading about substantially higher percentages in the past, but googling just now turned up a best guess of 2.4%. I couldn't find any details on how they calculated the indirect nature of defensive medicine, but what they did have didn't seem to be very comprehensive.