It all depends on how big a lifetime benefit you choose and the level of coverage you want. You can add many different options to basic plans.
But in general we are talking about an annual premium that will cost you a few thousand dollars a year, at a minimum. And you will pay that for a long time: from the first year of your policy all the way through until you start drawing on the policy. That could be 30 years down the line.
This is important: You will have to keep paying the premium when you are retired and probably living on a smaller income. If you stop paying your premium at any time, you can lose your coverage and every penny you paid up to that point. If you doubt your ability to keep paying for the policy through retirement, you probably shouldn't buy it.
The National Association of Insurance Commissioners (NAIC) suggests that you spend no more than 7% of your income on premiums.