That's going to be a matter of opinion and risk tolerance.
The cookie-cutter CFI policies, even those available through NAFI and SAFE have pretty low limits, typically $500,000 or $1 Million liability with $100,000 per seat limitations. That pretty much translates to $100,000 coverage for your student's injuries with the rest available only if you do substantial damage to persons and property on the ground. And to pay for a defense if a claim is made — that's probably it's greatest value.
Coverage for hull damage to the aircraft itself is separate - you choose how much coverage you want based on your risk calculation.
Cost is, perhaps surprisingly low, but may well reflect the real risk exposure. The typical policy is just a non-owned aircraft (renter) policy with a CFI coverage endorsement. The largest premium cost of the policy is the non-owned aircraft coverage, and the highest part of that is the hull coverage. The CFI piece is, as I recall about $150-$200.
Of course, risk calculation is very personal. What you find acceptable risk, I might think is sheer lunacy. And vice versa.
With that in mind, here's how I look at it personally. I've been comfortable with those low limits when teaching in someone else's airplane. I feel my highest risk exposure is when in the airplane teaching a student pilot. That one, to me, has a low chance of occurring although, if it does, the consequences could be staggering. I'm willing to accept that. For "CFI malpractice" claims, I think the risk of those are low overall and the costs and difficulties a plaintiff faces in bringing one (even in those states where the claim is viable) will likely result in acceptance of the policy limits . Again, a risk that is acceptable to me personally. I choose my hull coverage premium based on what I think is the most damage $-wise I might do to a rented (or client's) airplane.
I'll repeat the disclaimer: I give that only as an example. My personal risk calculation could very well change later today.