[Seeya]

One time purchase is silly without a trial gameplay, just make the game completely free to play with upgrades (Like exp bonuses, cash multiplier (1.5x money doing jobs or quicker mining/whatever)

could also have skins on your gear for money. Make the skins permanent, price them accordingly.

and perhaps a while after release you could offer larger bonuses, new areas to unlock for $$$ or ingame cash.