Depending on what size and if the insurance is through vzw or AppleCare. AppleCare is $50 I think.
$9.99 per month per covered device
$169 non-refundable deductible (8GB, 16GB iPhone® 4 and 16GB iPhone® 4S) per approved claim
$199 non-refundable deductible (32GB iPhone® 4 and 32GB, 64GB iPhone® 4S) per approved claim
2 Replacements in a 12-month period with an equipment maximum of $1500 per claim (in New York, 2 claims per policy year)
This includes extended warrenty.
You could either do that or buy the back glass from Amazon or something and get it installed by someone or do it yourself. Or find a tutorial on the internet. My friend had to do that. He shattered his front screen. And its cheaper. It also comes in different colors.