Judging by the relative square footage in our local Great*Wall*Mart (a very rough approximation at best, I'll grant you), I'll say about a quarter of total sales, or $50 billions.
But remember most of the Wallys aren't Super Wallys (only the Supers have fresh fruit), so that skews things a bit. Of course they're moving towards more, and building straight Wallys groceries now. Part of the problem with guessing how much of what Wally sells, they're a constantly moving target.