Was originally with AAMI, and noticed they had quietly dropped my covered rate to $7,500. If I was willing to pay more, I could be insured for $10,500 max. This is for a 2008 top spec Nissan 350z, which you couldn't look at for anything less than $17,000. My premiums for comprehensive were about to go up to $141 a month (despite being with them for 5 years with no claims) so I checked with Shannons, and they covered to $20,000 for $146 a month. Always pays to check how much you're covered, as if my car was written off, I'd be getting less than half it's worth. Outrageous.
Yeah, beware being insured for market value vs agreed value, especially if your car is in good condition, low kms etc.
I always play with the agreed value sliders, and for my car, it's little extra cost to have it insured for it's real value vs the 'average' market value.
Note also that the market value will usually continue to decline throughout the year. So, eg, using the example above, the market value might be $7500 now, but $6000 in another year.