The Mercenary clothing was a time-limited bonus, given with the purchase of a 1TB Seagate external Hard Disk Drive. If someone bought the HDD and has the DLC chances are they have either redeemed or sold it already.
If they haven't sold it yet and someone is willing to pay CA$90....then they will sell it for that much.
I'm not sure it is still in circulation officially, so, unfortunately, the price is set by who has it....and if people want to throw money at them, it can get ridiculous.
(and likely more so once there are fewer and fewer available)
It could be possible that it is only time-limited exclusivity, in which case should be available to buy in-game 'at some point'.