It's fine if you took my original statement as a direct offence, but it wasn't meant that way. Your argument was stupid, plain and simple. It wasn't supposed to be an attack on you, just your flawed argument. I previously explained part of why commercial cost more, further explanation is below. I didn't have the time or desire to deal with it further when I was at work. It came out at least partially wrong. However, that doesn't absolve you from being a complete fucking twat to me.
The simplest way to explain why you were wrong is that the insurance companies themselves charge more for commercial use. It sounds like a tautology, but it isn't. They base their rates on statistics. If the statistics say there is a higher risk of either more or larger payouts under commercial use, then there must actually be a higher risk. From what I can find, there is higher liability when a commercial entity is involved in an accident.