Probably the biggest factors when calculating travel insurance premiums is the length of the cover required. If you are taking a two-week vacation, there is a much lower risk than if you are making a yearlong voyage around the world. That said, there are many annual travel insurance policies that cover all the trips you make in a single year and if you make more than a couple of separate trips, annual protection may work out cheaper. Often family protection policies are not too much more expensive than a single persons policy so if you get the whole family insured together you are also likely to save.
Another big factor is where you are travelling. While some countries represent more health risks, as they are more dangerous, it is also a fact that poorer countries often have far lower medical costs so insurance tends to be highest for travellers in North America and Europe, with the US specifically recognised as having high health care costs. If you plan special high-risk activities such as skiing or mountain climbing you will either need separate insurance for this occasion, or to have the activity specifically added to your policy, as there is a good chance it will be...