You're missing a word, it should be "the bargain of a lifetime." A "bargain" can mean "an exceptional buying opportunity, a chance to buy something at a very low price." "Of a lifetime" means "once in a lifetime." "It was the thrill of a lifetime" means that of all the thrills you've ever had, this was the most exciting. "This is the bargain of a lifetime" means "this is the best bargain you'll ever see in your lifetime." Another way to say it is "This is a once-in-a-lifetime bargain." "For one day only, we're selling X at half price! This is the bargain of a lifetime!"

It means that the deal you are being offered is the best you will ever have in your whole life.