As far as I know, halakhically, you owe the Cohen 5 pieces of silver amounting to a total 102g. In my experience, you buy 5 silver coins from the Cohen and pay him whatever 102g silver is worth in a currency he accepts. If he accepts say BTC (you can calculate the exchange rate http://coinapps.com/bitcoin/silver/calculator/ for instance) transfer the required amount to the Cohen in exchange for the 5 silver coins, and then at the ceremony you give the 5 silver coins back to the Cohen. If the Cohen does not accept BTC you can convert it to anything else the Cohen does accept.
