Worldwide In-app Purchase (IAP) revenues reached almost US$37 billion in 2017 and are expected to double in 2020. Yet, only 5% of total app users make any IAPs and 70% of those in-app purchases come from big spenders or ‘Whales' who account for only the top 10% of the paying users. We investigate why users are unwilling to spend money on in-app purchases in mobile games with a multi-stage mixed-method study combining qualitative and quantitative approaches. We developed and validated a new construct of perceived aggressive monetisation, which explained user conversion concurrent with perceived fairness. A survey of 527 US and 526 Australian mobile gamers supported our conceptualisation of the IAP spending decision as a hurdle model with self-control and perceived aggressive monetisation as hurdles. However, once a user decided to spend money, the actual IAP spending is an impulsive mechanism explained by users' time-spent playing and exposure to marketing tactics. A follow up field experiment of 264 US mobile gamers show how the marketing tactics of app publisher can influence IAP spending for loot boxes -an infamous type of IAP- by manipulating the size of the offers, mode of currency, and informed probability for the loot box.