According to different cultures, regions and family backgrounds, when a cousin gets married, the amount of money that the youngest brother has to pay for being an uncle in the three dynasties may be different. Here are some possible scenarios:

1. According to the traditional custom, uncles of the Three Dynasties should give gifts to their cousins and grooms, usually some mascots or red envelopes. The value of gifts varies from region to region, generally ranging from 1000-5000 yuan.

If the youngest brother has already worked, he may give more money as a blessing, depending on his financial ability and personal wishes. It may range from several thousand yuan to tens of thousands of yuan.

3. If the cousin and groom have married, the uncles of the Three Dynasties may have to give some money to pray, which may range from several hundred yuan to several thousand yuan.

In short, how much money to give is not a fixed number, but is determined according to personal circumstances and traditional customs. It is important to express blessing and respect.