Team Fortress 2's business model has been fairly simple since it went free-to-play--sell lots of virtual hats and guns--but Valve's trying something slightly different with the new 'Mann vs. Machine' co-op PvE mode, which launched last night. While it's free to play, if you fancy extra special loot, you can pay 99 cents for a ticket which offers the chance to earn unique prizes the likes of which ye have never seen.
The 99-cent single-use Tour of Duty Tickets let you into Mann Up Mode, which is a fancy way of saying it's played on Valve's official Mann Co. servers and you can score snazzy robot parts to wear. You're given a wearable Tour of Duty Badge to track your progress through the three MvM missions, which levels up once you've finished them all. The first time you finish a mission on each badge level, you get a piece of robot-loot, then another piece once you've finished the entire tour.
It's not cheap, though. Tour of Duty Tickets are used up when you complete any mission not already finished on your badge level (but not if you fail or have finished it before on this level). Tickets are still ultimately a hat delivery mechanism, but a bit different to straight selling them. Look, it's complicated, so check Valve's FAQ for the nitty-gritty.
You can also cough up $1.99 for a Squad Surplus Voucher, which will ensure everyone on your Mann Up Mode team will get an item if you beat the mission.
Of course, you don't need to pay a penny to play Mann vs. Machine if you don't want to; player-run servers simply don't drop the special robot-loot. The items are tradeable too, so you can swap your spares for shiny newness if you don't mind being thoroughly fleeced by unscrupulous traders.
Team Fortress 2 is a little overwhelmed by the flood of people trying to smash robots and wear their metallic skulls as trophies, mind, so it may be a while before you can play.