I just moved back from the US - living in New York and Boston, I didn't have a need for a car and rented a fair bit.
As I think has been pointed out, there are two main types of insurance: loss damage waiver and supplemental liability. Unlike in Australia, if you don't have LDW there is no excess amount - you are typically liable for the value of the car. LDW reduces this amount to zero.
But don't forget the supplemental liability side of things, which is what you're liable for if you damage other people and their property. The standard liability insurance most rental cars come with is the minimum prescribed by the state of registration, and this is often as low as $25,000. This may well be enough in many cases, but in an extreme case you could be on the hook for quite a bit more than that if someone sues you. The supplemental liability insurance raises this substantially - I think it's $1,000,000 for Avis, for example. It's possible travel insurance covers this, but when I visited over Christmas I noticed the liability part of the travel insurance I got specifically excluded anything caused by operating a vehicle. Not sure if this is generally the case, but you'd need to carefully check the policy wording.
As for renting cars, I generally found the .com.au Avis and Hertz sites to have competitive prices in the NYC area at least. Their prices always included taxes, LDW and supplemental liability, and there were sometimes cases where I'd be shown a price of $80/day and the US site would show over $200 (plus taxes, and without any insurance). I also tended to find the .com.au prices were generally consistent throughout the year whereas the US sites varied much more by demand. If you're travelling at a busy time it definitely pays to book on the Australian sites.