There are many different currencies in the world. In order to ensure consistency, certain style codes should be recognized.
Prices should be listed with the currency symbol that travellers will encounter, specifically the local formatting. The currency symbol should always be prefixed. Travellers should be able to assume that symbols used for multiple currencies (like $ pr £) apply to the local currency. Do not use currency codes like "USD", "EUR", or "GBP".
If the currency name is short enough to be spelled out in full and/or lacks a commonly recognized symbol, it should come after the amount.
If the country uses multiple currencies, including foreign ones, use the shortest unambiguous form for each. For US dollars, this is US$. For euros, it's €.
Use a "." to mark decimals, and use a "," to separate thousands groups.
A billion is a thousand million (American style), not a million million (Commonwealth style).
When talking about the cost of an item in a country, stick to that country's currency. Do not switch between currencies. Doing so causes confusion and frustration. If you only know the price in dollars or euros, go to a currency conversion site and convert the number. Round off to nearest whole unit.
You can purchase a gift for Rs. 100. A taxi ride costs $10.
You can purchase a gift for Rs. 100. A taxi ride costs Rs. 450.