How much money?

What's a decent amount of cash to give a friend as a wedding present?

Depends where you live. In Holland 30 euro is fine.

Depends on the friend and if I know them both. Also depends on the reception, not that it matters a lot but some. $100 for a simple wedding and reception. $200-$500 for an elegant one depending on the venue.

I paid for my daughter's wedding in Mexico with a sit-down dinner reception on the beach. A mariachi band before dinner and a DJ after dinner for 50 guests. A week later I paid for a nice dress-up reception for the people that couldn't make it to the Riviera Maya at a nice venue near the house. Over 200 guests came to the second reception, but I never asked how much my daughter cash value collected from the money and gifts. My daughter wore her wedding dress, and we had a second father-daughter dance.