I've used Capture one since v.3 and every time the version number has incremented it has been a paid-for release. As is usually the case with software, the "dot" releases are free, but Capture One 7 - I'll bet pounds to pennies - will cost.
There is no secret here. Capture One upgrades
have been $99/license (allows 2 computers) for many years. Updates
are always free.
For those interested in the rather boring history of Capture One pricing:
Capture One 3 Pro was the exception. It was sold for many many years for $499 with "2 free upgrades". Phase One planned on making upgrades a 1-2 year cycle. But it took much longer to release Capture One 4 Pro (a complete rewrite with modern code). Anyone who paid for Capture One 3 Pro did not have to pay again through Capture One 5 Pro - for many customers that meant they did not pay for upgrades for 5+ years (while P1 was still paying to develop it). Because of this they decided to sell Capture One 4 Pro for $399 ($100 less) but without free upgrades associated with that purchase. Later the price was reduced to $299 (clear pricing pressure from Adobe LR) where it has been selling quite well. Lately there have been a string of promotions that reduce the price by varying amounts.
Also, anyone who made a purchase right before an upgrade has been given the upgrade for free. For instance if you bought Capture One 5 Pro two weeks before Capture One 6 Pro was released they gave you the upgrade for free.
Even if you're shooting a mid-tier dSLR if you shoot tethered or care at all about image quality and workflow speed then the cost of Capture One software (even with every update) is a very very small % of your equipment/software costs. Considering that for many photographers the software is where you are 50%+ of their total imaging time this seems to me to be the best ROI in photo spending.
Historically you've always been able to buy from a handful of dedicated dealers like us (Digital Transitions) for 5-10% less than from Phase One direct, in which case you gain phone/in-person support of that dealer for questions/issues/feature-requests in addition to retaining the email/online support of Phase One. I don't expect that to change whenever the next upgrade to Capture One is released.
And of course no one is forced to upgrade. If they bring enough to the table regarding new features, improvements, refinements etc to justify the cost then you will - if not, you won't.