How much is a 'perutah' worth in modern currency?
I believe for most Halachic topics today that require "one prutah's worth" of cash, goods, or services (e.g. the minimum needed to effect a wedding), we go with an economic definition: the smallest amount of money that can buy something. Despite inflation, there are plenty of things you can still buy today for fifty cents or less, so fifty cents is certainly shaveh prutah; I'm not sure whether it's a nickel, dime, or quarter, but fifty cents certainly is enough. Or as someone once said to my father:
You mean instead of a gold ring, a guy could march down the aisle and give the bride the little metal tab off his soda can?
My father replied:
No, that's not nearly enough. He'd have to give her the entire can of soda.
The weight of a p'ruta was half that of a barleycorn, and it was silver. Assuming — without justification — that barleycorns weigh now what they did then, and other unjustified assumptions, we have: Wikipedia gives the weight of a barleycorn as about 65 mg. At about 31,103.4768 mg per troy ounce and a current price of silver of about USD28.20, that gives about 2.9 cents to a p'ruta.