Credit card companies can afford to pay out huge bounties when you apply to get a FFP-branded credit card for a number of reasons. First, it's a big marketing "hook" -- and credit card companies spend a ton of money on advertising in order to acquire new customers. Second, some of the articles about recent transactions wherein some airline FFP's have sold their partner banks a huge chunk of miles upfront in order to improve the airline's liquidity situation have asserted that the average spend on a FFP-branded credit card is much higher than the average credit card. In which case, such customers are well-worth acquiring, since they generate profits for the credit card issuer both through fees paid by the merchants and through fees/interest paid by the percentage of customers who don't pay off their balances every month.

Therefore, I wouldn't consider the sign-up bonus miles that some credit cards are offering as evidence that the banks are able to buy miles at significantly less than a penny a mile.