Since your expected losses aren't in a lump sum, you can do some data analysis off a range of a few days and estimate the number of people that are dropping out: you can have a maximum of 333 participants and still break even, provided you're playing on blackjack with a 0.5% edge. Halve the amount (and then some) if you're going baccarat, and even more for roulette. 120 days x 5 mBTC x 1/37 edge is a whopping 16.21mBTC cost of participation, worth over $800 at current prices. Bear this in mind.

Incidentally, in blackjack games, you could argue that martingale is slightly more effective than flat-betting, since you are more likely to lose with excess low-cards, which would raise your odds in subsequent rounds.