I take the point @RobParker but the markets are completely different.
XenForo is able to set a relatively now price due to the number of licenses they sell and the renewal fee.
Third party add-ons will never approach the same volume.
Exactly. Yet customers don't appreciate that and so the XF price sets an artificial ceiling. There's been lots of instances on here of people saying they'd refuse to pay more than the core price for an addon.
(I'd happily pay £500+ for a decent addon that we really needed).