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 round-trip 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.