I worked for an "Authorized Retailer" so I can tell you how it works for them (and maybe draw some conclusions about corporate stores, too). They pay full price for the phone upfront. After six months of the customer paying their bill and keeping service on they get reimbursed for the cost difference by the carrier (or at least in my case, AT&T). Does that mean that AT&T paid full price also? I'm not sure. I just know that AT&T third party retailers (technically, Authorized Retailers are second tier only to Corporate Stores) pay full price up front for the device. I imagine this could have changed in the past few years, though.
If they count shipped as sold (like my experience above) then activations wouldn't tell us much. If HTC gets full price upfront for each device that leaves their assembly line, then they don't care if they ever get activated (we could go into buyback agreements for unsold devices, but that's beyond the scope of this discussion and my knowledge on the matter).