I've also been looking into this, and I got a quite from Coverdrone for my Air... £275, with a £300 excess. So if I crash or lose the drone, it'll be £575 I paid, and the drone costs what, £700? Not worth it.
FlyIcarus has come up with a little under £200 for the year, and a 10% excess (so £75). Not so bad, though still more than DJI refresh... but it does include fly away cover, so I guess it also includes loss in water (like losing it in a lake) neither of which is covered by DJI.
Tricky decision. I haven't found any reviews yet of Icarus where someone actually had a successful claim, which is the important part.
Edit: Icarus does not cover you if you fly over open water, so losing it in a lake is a no no still. Unless you lie and say it flew away, but they may want to see logs.
So it seems to be about the same as DJI refresh, but also covers you for theft and pubic liability. Not terrible.