I pay around £300-£400 a year (Lol having trouble remembering the exact amount).
That is with 'petplan equine' as I have used this company for all of my other horses in the past. I own a cob that I mainly do hacking and fun rides with- so thats quite simular, but he is also insured for driving on roads, they can adjust the insurance to your requirements i.e- riding/driving etc.