I work in the insurance industry and although you can get a cheap quote you will have to check the exclusions on the policy as most of them will exclude countries where the FCO advises against travel to.
I have arrange my insurance through Battleface. They do 2 levels of cover. I'm just went for the medical expenses cover and for me and my daughter for 1 week it was just over £100. Worth every penny for peace of mind.
An update on Tunisia travel and medical insurance, battleface, April insurance and versatile are all offering a cover that includes against terrorism now. Pricey but better to be safe than sorry. Happy travels folks x