The theory is, the Sixers will not be able to outbid the Lakers or Knicks for any free agent on any four-year deal, and might not even be able to keep Butler if not for the fact that they can give him five (Butler is even making noise already that he doesn't care about that, which is certainly leverage).
And the same probably goes for Harris, even if he seems like less of a no-brainer max deal player, someone out there will probably give him four.
And they have no trade assets (barring sign-and-trade I guess. Or Simmons).
So you can't measure Butler and Harris against the $, you can only measure them against the cheaper and likely lesser options available to the Sixers in free agency instead. But, they also might lose Reddick.
So, there's probably a case to be made for ditching Harris in favor of some combination of Reddick, a better-paid back-up center and a cheap 5th starter, but it's not clear-cut, and you might lose Harris before you know who you can actually get.