For reaching tier 2 you would have to wager 0.005 BTC. If you make many bets to reach this amount then you will likely end up losing an amount that corresponds to the house edge which is about 0.00025, which isn't too devastating. If you only make a few bets the results can be more volatile but if you use a low multiplier then you are more likely to have a positive result. The only time I've gone for the flash promo bonus I made fewer than 10 bets with a 1.01 multiplier and won all of them.