The currency of Paraguay is the Guarani (PYG). In the capital of Paraguay – Asuncion, credit cards can be used and there are ATMs. If your tour of Paraguay takes you into more remote parts of the country, especially the Chaco, cards cannot be relied upon and you need to travel with cash.
How much money you need for your trip to Paraguay will depend on your tour itinerary. The main additional costs are likely to be meals and money for any shopping you plan to do.