You can buy different jetpacks by collecting coins and then going into the Stash. The cheapest you can get this achievement is for 15,000 coins by buying the Bubble Gun Jetpack for 7,000 and the Steam Powered Jetpack for 8,000.
However I would suggest getting the Traditional Jetpack for 10,000 instead. The Traditional Jetpack is needed for the Class Act achievement:
Bubble Gun Jetpack - $7000
Steam Powered Jetpack - $8000
Traditional Jetpack - $10000