The purchase options are pretty basic, only having the ability to purchase x amount of tokens for x amount of real money.
Why not have 'special' offers ie 100 tokens + 10 engines for x amount or 100 tokens at half price for the next 24hrs etc. Most other pay games run promotions quite often, yet here I have only seen 1 promo since June and that was upon the release of the latest update.
You could also run a tier scheme of different vip levels so to become VIP2 you need to have spent x amount and the reward for VIP level 2 would be 1 extra token and 1 engine per day etc.