It really is worth having a good Google around and taking a number of quotations - the cost and cover can vary quite widely, depending on what you want to include in the policy.
Insurance against anything going wrong ie the venue, the photographer etc. Because weddings cost so much these days it's worth taking out insurance in case anything goes wrong and I don't mean the groom not turning up!
I'd think hard about what else you could do to mitigate these things. No venue, no wedding for example. and no amount of money will repay you or the unhappy couple if the photographer loses his camera. Is it really worth the premium?