Still trying to find the cheapest way to fly business class from CPT to LHR

Is it generally cheaper to just buy a business class ticket outright, than to buy an upgradeable economy and then upgrade at checkin? Also less risk of there not being space to upgrade, I supoose....

I know a few years ago I'd booked economy and then got an email offering me a ridiculously cheap upgrade, but I would think that's not the norm