I went to the most known prison here in the Philippines and the people there probably have more money than a typical minimum wager outside the bars. Of course, the leaders or called 'mayors' and 'commanders' have more power and it means money as well. But if there are slaves, there are also there. Looks like that gambling inside is really prohibited but with some under the table to the jail warden and whoever is guarding them on their bars, they'll just bribe them with money and every rounds of pots/wins, there is 'tong' or tax type in gambling whether they play some alternative casino games or card games.