Actually BakDirect gives you one tenth of a mile per dollar per month. So if you have $25k for 6 months, you get 2500 miles per month, for a total of 15,000 miles. Guaranty Bank works the same way. (Both of these are tied to AAdvantage.)

It's still not a "good investment" compared to the money-market accounts, but it's not completely ridiculous as it would be if it were paying one hundredth of a mile per dollar per month as your example suggests.