Someone can tell us in a usability test when they're confused by a page or when they're frustrated by upsells during the checkout process. They probably can't tell us whether one set of copy or another is more likely to make them feel an affinity with our landing page, pull out their wallet, and plunk down cash.
So, we A/B test landing pages and payment flows.
We don't A/B test price. Users talk to each other and it can make customer support difficult. We test the price off-line via customer interviews instead.