The way Adobe structures their pricing, the "assumption" is that most people using Team projects will be in companies with Enterprise accounts. I think that was ... not wise on their part ... but that's the reality.
Team projects workflow does require a lot more storage and net bandwidth, ergo they do want an additional fee. It works out to around $20 USD more per month than a "base" CC license. In the US, currently, around $50 for a base license and $70 for adding on a Team Projects licence.