C-Bills

C-Bills are an in-game currency obtained with real world money - either purchased directly or received as a monthly stipend for premium accounts.

Some store items will be purchasable with either C-Bills or scrap, others will be purchasable with C-Bills only.

The Founder FAQ puts the cash-to-C-Bill exchange rate as 1 USD = 100k C-Bills

According to the Penny Arcade coverage, "lower level" (most likely Bronze) STACs will cost approximately 1 USD in real currency, with "highest-level" ones (ie Platinums) costing under 5 USD.

Together with the Scrap prices shown on the Store screenshot in the PA article, this indicates the theoretical C-Bill price of a Bronze STAC ( shown as costing 1000 Scrap ) would be around 100k C-Bills - which leads to a theoretical exchange rate of 1 USD = 100k C-Bills = 1000 Scrap.

While the Scrap price of the Platinum STAC in the screenshot leads to an exchange rate which is not consistent with the above calculation, later screenshots show Platinum STACs priced at 5000 Scrap (and thus validating that theory)