Don't do this! By all means give a donation to the Wat, but the Buddha didn't do this, no Western wats do this, and basically it's a modern Thai way of making cash for the monks presiding at the ceremony.
In the past I've seen this, and the senior monks divide the cash up between them. It does NOT go to the temple fund, it goes straight into the pockets of the monks. The upachaya gets most, and the lesser ones according to status.
And do not handle money as a bhikkhu! If you do this, you are corrupt. Not handling money is one of the main ways in which a bhikkhu is different from a layperson.
If you need to handle money, then the conditions are not right for you to be a bhikkhu. Yes, i understand 95% of Thai monks handle money but that doesn't make it right. It just shows how far genuine Buddhism has fallen in Thailand.