You can buy Crown items from the Store, and then by selecting the class you would like to buy an item from (Rifleman, Medic etc.) The cheapest Crown item currently available is the Crown Gloves which can be found under any of the four classes. Once you pick a class, press 8 times to reach the Gloves section and you will see the Gloves, costing 2100 Crowns. This means, assuming you only earn your crowns through completing Insane contracts, you will need to complete those contracts for 42 days.
Note: Buying the gloves will unlock the achievement for you, but the gloves themselves will only be available to you for 14 days, as stated in-game it still only a rental item costing 2100 Crowns.