Problem with insurers is that they do their calculations based on probability, revenue generated from premiums and the cost of claims.
The aerobatic 'market' is very small and the probability of a claim is fairly unknown (whilst we'll all argue it's very safe there simply isn't enough activity to be able to give what insurers consider an accurate assessment). End result is that insurers will assume a high risk and will load your premium accordingly. Seem to remember a quote I had a couple of years ago was several hundred pounds per month for life cover - but then I did mention gliding, aerobatics, competitions and instructing all in the same sentence!.
Best bet is to google 'extreme sports insurance' and see if you can get aerobatics covered.