That's not quite what I asked. I asked which parts don't scale with time/compensation, which is the only bit that matters for shorter-than-full-time schedules.
Google developers start at $191k per year. The market price of Platinum health care plans for a 30-year-old single person in Google's Zip code is ~$1000/month. So that's ~6%.
That's nothing to sneeze at, but it's also not huge. If they drop to an 80% schedule and 80% comp, health care's now ~8% of their salary. That's something that Google can surely handwave away.
For companies that can't, I think there are reasonable solutions. The obvious one is that they scale their health care subsidy with the hours worked. In many places employees pay a percentage of their health insurance costs already; it's not hard to change those numbers for people on less-than-full-time schedules.