Yes, spare money that you can afford to let be at stake. I mean, if you have a good amount of spare money and you are willing to invest it for long term, BFG is one of those unique assets that you can use. The business is continuing to progress and the support from players and traders is really something that can bring the coin into more success.
Though it's always depends on how you as investors understand both the risk and the possible gains.
More on your own take and how will you trust the big potential of this passive investment?
There is a calculator on Betfury that you can use so it should be pretty easy to determine the ROI. I prefer just to collect more tokens myself by playing on the casino itself; it gives you fun and also gives you more tokens.