How much $$ should I give to casual friends at a wedding

There is no set amount to give at any wedding. Take several factors into consideration:your finiancial situation, how large is the wedding,how well do you know the couple. I think no less than $100 and up to?

About £50. Don't know what that is in us dollars.

If you bought them a gift, probabley it would work out cheaper,you could pretty it up and make it look a lot more than what it actually is.
Very often you see what you think is a beautiful gift, which later turns out to be lots of lovely packaging.