Two things...
1: Jon said at the starting line of Season 3 that this year's prize totaled almost $1 million, unless I misheard him.
2: When the Bransen Family won "free gas for life" back in season 8, they revealed later that there were quite a few stipulations to this, and that "for life" was just an attractive little label that didn't really mean "forever, infinitely until you die".