yes, they put cash in the red envelope.
there is no such thing average, it really depends on how much your parents-in-law ask for or how much can you afford. usually it is a lucky number
not only cos of 手緊.
The traditional way, 女家客人俾嘅利是就女家袋. And same for 男家. (which ever side's 利是 goes to that side) and the 男家 also pay for the 酒席.
But now, I think 禮金 is just a symbol. So just a small amount is ok.
Most couple pay for their own wedding anyway.