Earn 50,000 bonus miles when you sign up for the new Frontier Airlines World MasterCard. You'll earn 40,000 miles when you spend $500 in the first 90 days, and up to 10,000 miles on balance transfers (1 mile per $1) in the first 90 days. This means you can get 50,000 miles by only spending $500, one of the lowest spending thresholds we've seen for 50k miles! Better yet, domestic roundtrip tickets start at only 20,000 miles. You'll also earn 2x miles on Frontier Airlines purchase and 1 mile per dollar on everything else. There's a $69 annual fee.