Fastest: Coinmama There are two main things that can slow down buying cryptocurrency. It works on both mobile and desktop, it offers a range of payment methods to suit almost anyone, and it walks buyers through each step to making their first purchase. To find the cheapest exchange, we looked at overall cost-effectiveness when buying BTC with fiat currency, including exchange commission fees, deposit and withdrawal fees, exchange costs and any other applicable fees. Each individual Bitcoin is divisible to 0.