The actual price of 1 Credit depends largely on how you buy them, more is cheaper in relation. (10Cr = €5 => 1Cr = €0.5, or 50Cr = €22.5 => 1 Cr = €0.45).
But let's assume you bought a 10-pack .. You've played some game world for 3 weeks, which is 5+1+1 Cr (5 to join and first week, then 1 per week), and then join another world, 'costing' you another 5Cr .. Naturally you "lose" there 4 weeks (or 4Cr, or €2) of play time in the other world with the same Credit amount if you leave the other world alltogether and join a second world instead. As the Cr system is made so that the longer you stay in a game world the cheaper it will be for a single day, which s natural - overall the game cost is indeed the about €0.07 - €0.14 / day (again depending on how long you play in a world, and how you bought the credits etc, but averaging about 0.1€/day). So jumping back and forth between worlds isn't that recommended.
But life's full of choices ... And you can also play in many worlds at the same time too.