I´d advise paying on the metro as there´s a fine of 101 euros if you´re caught without a ticket, as we might have been but for a bit of deft dodging involving a map and an "oh it must be that way" demeanour.
Updates
Buy a one month ticket for 30 Euros and travel everywhere within reason. Newspaper stands near the stations have them but they are for a calendar month. This is great value.
Is this your business?