Metal Gear Solid Acid....Help!

I recently bought a psp with Mgs Acid.

I understand everything bar one thing....cards have a cost....and this cost gets added up....but....whats the point in it?

It says when cost reaches 0 you have another go but the cards you use all have numbers on and this get added up...is there a limit at which the cost resets itself???

I just don't get it.

I've read the manual too. It just says there's a cost.