Its just the way insurance works, the higher the risk you are, the higher the premium. Even though you have survived one heart attack, you are at a greater risk of a further one, so up goes the premium. Just because you are now taking medication for the problem doesnt mean it wont happen again unfortunately.
My mum and her friend actually stopped going abroad because of the huge premiums due to a heart problem, they simply refused to pay such huge sums on top of the £1500 cost of the holiday, and I dont blame them.
All you can do is shop around as much as you can, and hopefully you will find a more reasonably priced one.
Good luck!!